JP6746625B2 - Optical fiber - Google Patents

Description

The present invention relates to an optical fiber whose core can be expanded by thermal diffusion.

Semiconductor optical waveguides typified by silicon optical waveguides are highly expected as a technology that contributes to the integration of optical communication devices. For example, silicon waveguides having functions such as an optical modulator, a photodetector, and an optical switch have been realized, and their use in optical communication has begun.

A CSMF (Conventional Single Mode Fiber) is often connected to the semiconductor optical waveguide in order to propagate the light input to the semiconductor optical waveguide or the light output from the semiconductor optical waveguide. However, the mode field diameter of the semiconductor optical waveguide is about 1 μm, whereas the mode field diameter of CSMF is about 10 μm. For this reason, if the CSMF is butt-connected to the semiconductor optical waveguide, the connection loss due to the difference in the mode field diameter becomes too large and it cannot be put to practical use.

Therefore, there has been proposed a method of forming an SSC (Spot Size Converter) in the semiconductor optical waveguide and connecting a CSMF to the SSC (see Patent Document 1). However, if the mode field diameter of the semiconductor optical waveguide is expanded to the same extent as the mode field diameter of CSMF by using SSC, there is a problem that the loss in SSC becomes large. Therefore, one end of a bridge fiber having a mode field diameter of 4 μm is butt-connected to a semiconductor optical waveguide whose mode field diameter is expanded to 4 μm by using SCC, and the other end of the bridge fiber has a mode field diameter of 10 μm. It is considered to fusion-splice.

As such a bridge fiber, an optical fiber having a TEC (Thermally Diffused Expanded Core), that is, an optical fiber whose core can be expanded by thermal diffusion is used (see Patent Documents 2 and 3). Then, the core of the bridge fiber expands during fusion with the CSMF or by subsequent heating, so that the mode field diameter mismatch between the bridge fiber and the CSMF is reduced, and as a result, the fusion between the bridge fiber and the CSMF is reduced. This is because the connection loss at the landing point can be reduced.

The expansion of the core by thermal diffusion is caused by up-dopants (additives for increasing the refractive index of quartz glass) added to form the core being diffused to the surroundings by heating. When the updopant added to form the core is germanium (Ge), it is known that co-addition of germanium, phosphorus (P), and fluorine (F) around the core can promote the expansion of the core. (See Patent Document 4).

Patent No. 5900484 (issued April 6, 2016) JP-A-3-130705 (published on June 4, 1991) JP-A-2003-75677 (Published March 12, 2003) Patent No. 3993198 (issued October 17, 2007)

However, with respect to an optical fiber whose core is composed of silica glass doped with germanium and whose inner cladding is composed of silica glass doped with germanium, phosphorus, and fluorine, it is only necessary to add these dopants at known concentrations. The inventors of the present application have found that the connection loss at the fusion-bonding point with does not become sufficiently small (for example, does not become 0.2 dB or less). This means insufficient expansion of the core due to thermal diffusion.

The present invention has been made in view of the above problems, and an object thereof is an optical fiber in which a core expands by thermal diffusion, and a connection loss at a fusion point with CSMF is sufficiently small (for example, It is to realize an optical fiber (less than 0.2 dB).

In order to solve the above problems, an optical fiber according to the present invention is a core made of silica glass to which an updopant is added, and an inner clad that covers the side surface of the core, both an updopant and a downdopant. An inner clad made of silica glass to which is added, and an outer clad that is an outer clad that covers the outer surface of the inner clad and is made of quartz glass, and the refractive index of the inner clad is , The refractive index of the outer cladding is substantially equal, and the concentration of the updopant in the inner cladding is determined so that the rate of increase in the refractive index due to the updopant is 0.25% or more and 0.5% or less. It is characterized by

According to the above configuration, when the optical fiber is fusion-spliced to another optical fiber (for example, CSMF), the core can be sufficiently expanded in the vicinity of the fusion point, and as a result, at each fusion point. The connection loss can be made sufficiently small (for example, 0.2 dB or less).

In the optical fiber according to the present invention, the concentration of the updopant in the core is preferably set so that the mode field diameter at a wavelength of 1550 nm is 3.5 μm or more and 6.5 μm or less.

According to the above configuration, when the optical fiber is butt-connected to the semiconductor waveguide, the connection loss can be suppressed to be small while maintaining the tolerance against the axis deviation.

In the optical fiber according to the present invention, the concentration of the downdopant in the inner cladding is determined so that the absolute value of the relative refractive index difference of the inner cladding with respect to the outer cladding is 0.1% or less. Is preferred.

According to the above configuration, the cutoff wavelength, the bending loss, etc. can be brought close to a desired value (a value expected when the refractive index of the inner cladding and the refractive index of the outer cladding match). .. If the refractive index of the inner cladding is larger than that of the outer cladding, the cutoff wavelength becomes long, which may cause a problem that the single mode operation becomes difficult.

In the optical fiber according to the present invention, it is preferable that germanium is added to the core as the up-dopant and fluorine is added to the inner cladding as the down-dopant.

According to the above configuration, when the optical fiber is fusion-spliced to another optical fiber, the core can be further expanded in the vicinity of the fusion point, and as a result, the connection loss at the fusion point can be further reduced. You can

In the optical fiber according to the present invention, for example, one or both of phosphorus and germanium may be added to the inner cladding as the updopant. Further, in the optical fiber according to the present invention, boron may be further added to the inner cladding as the down dopant.

It is preferable that the optical fiber according to the present invention further includes a pair of stress applying portions symmetrically arranged with respect to the core.

According to the above configuration, the optical fiber can function as a polarization maintaining fiber.

In the optical fiber according to the present invention, the pair of stress applying portions are arranged such that their outer edges are in contact with the outer edge of the inner clad, or their outer edges are separated from the outer edge of the inner clad, It is preferable.

According to the above configuration, it is possible to reduce the non-circularity of the core, that is, increase the circularity of the core.

An optical device including the optical fiber, a semiconductor optical waveguide butt-connected to one end of the optical fiber, and a CSMF (Conventional Single Mode Fiber) fusion-bonded to the other end of the optical fiber, Within the scope of the present invention.

According to the present invention, it is possible to realize an optical fiber whose core can be expanded by thermal diffusion and which has a sufficiently small connection loss at the fusion point with CSMF.

It is a figure which shows the structure and refractive index distribution of the optical fiber which concerns on embodiment. The upper stage is a cross-sectional view showing the structure of the optical fiber, and the lower stage is a graph showing the refractive index distribution of the optical fiber. 2 is a graph showing the concentration distribution of updopant added to the core in the optical fiber shown in FIG. 1. (A) shows the concentration distribution before the heat treatment, (b) shows the concentration distribution during the heat treatment, and (c) shows the concentration distribution after the heat treatment. 2 is a graph showing a concentration distribution of an updopant added to the inner cladding in the optical fiber shown in FIG. 1. (A) shows the concentration distribution before the heat treatment, (b) shows the concentration distribution during the heat treatment, and (c) shows the concentration distribution after the heat treatment. 2 is a graph showing the concentration distribution of downdopant added to the inner cladding in the optical fiber shown in FIG. 1. (A) shows the concentration distribution before the heat treatment, (b) shows the concentration distribution during the heat treatment, and (c) shows the concentration distribution after the heat treatment. It is a graph which shows the refractive index distribution of the optical fiber shown in FIG. (A) shows the refractive index distribution before the heat treatment, (b) shows the refractive index distribution during the heat treatment, and (c) shows the refractive index distribution after the heat treatment. 7 is a graph showing the correlation between the refractive index increase rate Δ due to the updopant added to the inner cladding and the splice loss at a wavelength of 1550 nm in the optical fibers according to Examples and Comparative Examples. It is a figure which shows the structure and refractive index distribution of the optical fiber (polarization maintaining fiber) which concerns on a 1st modification. It is a figure which shows the structure and refractive index distribution of the optical fiber (polarization maintaining fiber) which concerns on a 2nd modification. (A) is sectional drawing of the base material of the optical fiber (polarization maintaining fiber) which concerns on a comparative example, (b) is sectional drawing of the same optical fiber after drawing. It is a top view which shows the structure of an optical device containing the optical fiber shown in FIG.

(Structure of optical fiber)
The structure of the optical fiber 1 according to the embodiment of the present invention will be described with reference to FIG. The upper part of FIG. 1 is a cross-sectional view showing the structure of the optical fiber 1, and the lower part of FIG. 1 is a graph showing the refractive index distribution of the optical fiber 1.

As shown in the upper part of FIG. 1, the optical fiber 1 covers a core 11 having a circular cross section, an inner clad 12 having a circular cross section that covers the side surface of the core 11, and an outer surface of the inner clad 12. , An outer cladding 13 having an annular cross section. The optical fiber 1 may further include a protective coating layer (not shown) having an annular cross section, which covers the outer surface of the outer cladding 13.

In this embodiment, the diameter d1 of the core 11 is 4 μm, the diameter (outer diameter) d2 of the inner clad 12 is 16 μm, and the diameter (outer diameter) d3 of the outer clad 13 is 80 μm. Here, the diameter d3 of the outer cladding 13 is set to 80 μm in order to ensure the reliability regarding the mechanical strength even in a bent state. The outer cladding 13 may have a diameter d3 of 125 μm depending on the purpose of use and environment of use.

The core 11 is made of quartz glass to which germanium (Ge) that is an updopant is added. Therefore, the refractive index n1 of the core 11 is higher than the refractive index n3 of the outer cladding 13 (substantially the same as the refractive index of pure silica glass as described later), as shown in the lower part of FIG. In the present embodiment, in order to set the mode field diameter at the wavelength of 1550 nm to 3.5 μm or more and 6.5 μm or less, the relative refractive index difference Δ1={(n1−n3)/n1}×100 of the core 11 with respect to the outer cladding 13. Is set to 1.0% or more and 2.8% or less, and the concentration of the updopant added to the core 11 is determined.

In the present embodiment, germanium is added to the core 11 as an updopant, but the present invention is not limited to this. That is, a configuration in which phosphorus (P) is added as an updopant to the core 11 instead of germanium may be adopted, or a configuration in which phosphorus is added to the core 11 as an updopant in addition to germanium may be adopted.

The inner clad 12 is fluorine (F), which is a down dopant for promoting diffusion of the up-dopant added to the core 11, and germanium (Ge), which is an up-dopant for canceling the decrease in the refractive index due to the down-dopant. It is made of quartz glass to which phosphorus and phosphorus (P) are added. Therefore, the refractive index n2 of the inner cladding 12 is substantially the same as the refractive index n3 of the outer cladding 13, as shown in the lower part of FIG. In the present embodiment, (1) the concentration of the updopant added to the inner cladding 12 is set so that the refractive index increase rate Δ due to the updopant is 0.25% or more and 0.50% or less, and (2) The concentration of the downdopant in the inner cladding 12 is determined so that the absolute value of the relative refractive index difference Δ2={(n2-n3)/n2}×100 of the inner cladding 12 with respect to the outer cladding 13 is 0.10% or less. ing. Here, the refractive index increase rate Δ due to the updopant is defined as n is the refractive index of the silica glass before the updopant is added, and n′ is the refractive index of the silica glass after the updopant is added, It is an amount defined by {(n'-n)/n'}*100.

In addition, in the present embodiment, a configuration in which fluorine is added to the inner cladding 12 as a down dopant is adopted, but the present invention is not limited to this. That is, a configuration may be adopted in which boron (B) is added as a down dopant to the inner cladding 12 instead of fluorine, or a configuration in which boron is added as a down dopant in addition to the inner cladding 12 in addition to fluorine. Good. Further, in the present embodiment, a configuration is adopted in which both germanium and phosphorus are added to the inner cladding 12 as updopants, but the present invention is not limited to this. That is, a configuration may be adopted in which only germanium is added to the inner cladding 12 as an updopant, or a configuration in which only phosphorus is added to the inner cladding 12 as an updopant.

The outer cladding 13 is made of quartz glass to which a dopant other than chlorine (Cl) has not been intentionally added. That is, since neither up-dopant nor down-dopant is added to the silica glass forming the outer clad 13, the refractive index n3 of the outer clad 13 is substantially the same as the refractive index 1.46 of pure silica glass. ..

As described above, the inner cladding 12 contains a sufficient amount of down-dopant (a sufficient amount to offset the refractive index increase rate Δ of 0.25% or more due to the up-dopant added to the inner cladding 12). The down-dopant acts to lower the refractive index of the core 11 by itself diffusing into the core 11, and promotes diffusion of the up-dopant added to the core 11 into the inner clad 12 to promote the diffusion of the inner clad 12. It has the effect of lowering the refractive index. Therefore, when the optical fiber 1 is fused with another optical fiber, the diameter d1 of the core 11 can be sufficiently increased in the vicinity of the fusion point, and the connection loss at the fusion point can be sufficiently suppressed.

In particular, the fluorine added to the inner clad 12 remarkably promotes the diffusion of germanium added to the core 11. Therefore, as in the present embodiment, by adopting a configuration in which germanium is added to the core 11 as the up-dopant and fluorine is added to the inner cladding 12 as the down-dopant, the diameter d1 of the core 11 near the fusion point is It can be remarkably expanded and the connection loss at the fusion point can be remarkably suppressed.

When the relative refractive index difference Δ1 of the core 11 with respect to the outer cladding 13 is smaller than 1.0%, the mode field diameter at a wavelength of 1550 nm is larger than 6.5 μm. For this reason, when the optical fiber 1 is connected to the silicon waveguide, the connection loss becomes large. On the other hand, when the relative refractive index difference Δ1 of the core 11 with respect to the outer cladding 13 is larger than 2.8%, the mode field diameter at the wavelength of 1550 nm is smaller than 3.5 μm. For this reason, when connecting the optical fiber 1 to the silicon waveguide, alignment becomes difficult (tolerance with respect to axis deviation becomes small). Therefore, when the optical fiber 1 is connected to the silicon waveguide by setting the relative refractive index difference Δ1 of the core 11 to the outer cladding 13 to 1.0% or more and 2.8% or less as in the present embodiment. The loss can be suppressed to be small and the alignment can be facilitated.

The concentration of the down-dopant added to the inner clad 12 is indirectly defined by using the concentration of the up-dopant added to the inner clad 12 and the relative refractive index difference Δ2 of the inner clad 12 with respect to the outer clad 13. This is because it is difficult to directly measure the concentration of fluorine, which is the down dopant added to the inner cladding 12.

(Expansion of core in optical fiber)
Next, the core expansion that occurs when the optical fiber 1 is fused to another optical fiber will be described.

Each dopant added to each part of the optical fiber 1 is diffused by the heat treatment. When the dopant concentration distribution u(r,0)=δ(r) at time 0, the dopant concentration distribution u(r,t) at time t is given by equation (1).

D in the equation (1) is defined by the equation (2) and is called a diffusion coefficient. In equation (2), Q is the activation energy, R is the gas constant, T is the absolute temperature, and D ₀ is the empirical constant.

FIG. 2 is a graph showing the concentration distribution of the updopant (germanium) added to the core 11. In FIG. 2, (a) shows the concentration distribution of the updopant before the heat treatment, (b) shows the concentration distribution of the updopant during the heat treatment, and (b) shows the updopant of the updopant after the heat treatment. The concentration distribution is shown. In FIG. 2, r1 is the radius of the core 11 before the heat treatment, and r2 is the radius of the inner cladding 12 before the heat treatment.

According to FIG. 2, by the heat treatment, the updopant that was unevenly distributed in the region where r<r1 (core 11 before heat treatment) diffuses into the region where r>r2 (inner clad 12 before heat treatment). I understand that.

FIG. 3 is a graph showing the concentration distribution of the updopant (germanium and phosphorus) added to the inner clad 12. In FIG. 3, (a) shows the concentration distribution of the updopant before the heat treatment, (b) shows the concentration distribution of the updopant during the heat treatment, and (c) shows the updopant of the updopant after the heat treatment. Indicates the concentration. In FIG. 3, r1 is the radius of the core 11 before the heat treatment, and r2 is the radius of the inner cladding 12 before the heat treatment.

According to FIG. 3, the updopant that was unevenly distributed in the region where r1<r<r2 (the inner cladding 12 before the heat treatment) becomes r<r1 (the core 11 before the heat treatment) and r>r2. It can be seen that the asymmetric diffusion occurs in the region (outer clad 13 before the heat treatment). The reason that the updopant diffuses asymmetrically, that is, the diffusion to the region where r<r1 is suppressed is more suppressed than the diffusion to the region where r>r2, is because the updopant is added to the region where r<r1. It is considered that the updopant is not added to the region where r>r2.

FIG. 4 is a graph showing the concentration distribution of the down dopant (fluorine) added to the inner cladding 12. In FIG. 4, (a) shows the concentration distribution of the downdopant before the heat treatment, (b) shows the concentration distribution of the downdopant during the heat treatment, and (c) shows the downdopant of the downdopant after the heat treatment. Indicates the concentration. In FIG. 4, r1 is the radius of the core 11 before the heat treatment, and r2 is the radius of the inner cladding 12 before the heat treatment.

According to FIG. 4, the down-dopant distributed unevenly in the region where r1<r<r2 (the inner cladding 12 before the heat treatment) becomes r<r1 (the core 11 before the heat treatment) and r>r2. It can be seen that the diffusion is performed substantially symmetrically in the region (the outer cladding 13 before the heat treatment) (the diffusion symmetry is higher than that of the updopant). It is considered that the reason why the down-dopant diffuses substantially symmetrically is that the down-dopant is not added to the region where r<r1 or the region where r>r2. Therefore, the amount of down-dopant diffused from the region of r1<r<r2 to the region of r<r1 is larger than the amount of up-dopant diffused from the region of r1<r<r2 to the region of r<r1. Also increases.

FIG. 5 is a graph showing the refractive index distribution of the optical fiber 1 estimated from the distributions of the respective dopants shown in FIGS. 2, 3 and 4. In FIG. 5, (a) shows the refractive index distribution before the heat treatment, (b) shows the refractive index distribution during the heat treatment, and (c) shows the refractive index distribution after the heat treatment. In FIG. 5, r1 is the radius of the core 11 before the heat treatment, and r2 is the radius of the inner cladding 12 before the heat treatment.

According to FIG. 5, it is confirmed that the region having a relatively high refractive index, which functions as the core, is expanded by the heat treatment.

(Example)
Hereinafter, the results of actually measuring the connection loss at the fusion point with CSMF for the optical fibers A to D according to the example and the optical fibers E to G according to the comparative example will be described.

As the optical fibers A to G, optical fibers having a core diameter of 4 μm, an inner cladding diameter of 16 μm, and an outer cladding diameter of 80 μm were prepared. In each of the optical fibers A to G, the up-dopant added to the core is germanium, the down-dopant added to the inner cladding is fluorine, and the up-dopant added to the inner cladding is germanium and phosphorus. Is. The concentration of germanium added to the core is determined so that the relative refractive index difference of the core with respect to the outer cladding is +1.0% or more and +2.8% or less, and the concentration of the updopant added to the inner cladding is The absolute value of the relative refractive index difference of the inner clad with respect to the outer clad is determined to be 0.10% or less. As the CMSF, an optical fiber having a cladding diameter of 125 μm and a mode field diameter of 10.6 μm at a wavelength of 1550 nm was prepared.

First, the concentration of each updopant added to the inner cladding of each optical fiber A to G was measured using EPMA (Electron Prove Micro Analyzer). Further, the refractive index increase rate Δ due to the updopant added to the inner cladding of each of the optical fibers A to G was calculated from the measured concentration of each updopant according to the method described in Non-Patent Document 3. For each of the optical fibers A to G, the measured concentration of each updopant is shown in the second and third columns of Table 1, and the calculated refractive index increase rate Δ is shown in the fourth column of Table 1.

Next, each optical fiber A to G was fused to CSMF, and the connection loss at the fusion point was measured. The fusion of the optical fibers A to G and the CMSF was performed as follows. That is, after the end faces of the optical fibers A to G and the CMSF are smoothed by a fiber cleaner, the smoothed end faces are connected by an arc discharge type fusion splicing device (specifically, FSM-100P manufactured by Fujikura Ltd.). Fused together. Further, the measurement of the connection loss at the fusion point was carried out as follows. That is, while performing a heat treatment by arc discharge in the vicinity of the fusion point, the measurement of the transmission loss at a wavelength of 1550 nm was repeated, and the connection loss was calculated from the minimum value of the measured transmission loss. The measured connection loss for optical fibers A to G is shown in the fifth column of Table 1.

FIG. 6 is a graph showing the correlation between the refractive index increase rate Δ due to the updopant and the splice loss at a wavelength of 1550 nm. According to the graph shown in FIG. 6, when the refractive index increase rate Δ due to the updopant is 0.25% or more and 0.50% or less, the connection loss at the wavelength of 1550 nm can be suppressed to 0.2 dB or less. This is because a sufficient concentration of downdopant is added to the inner cladding to offset the increase in refractive index due to the updopant, which promotes diffusion of the updopant from the core to the inner cladding, resulting in sufficient core expansion. It is thought that this is because it can be achieved.

When the refractive index increase rate Δ due to the updopant is smaller than 0.25%, the connection loss at the wavelength of 1550 nm exceeds 0.2 dB. Further, when the refractive index increase rate Δ due to the updopant is larger than 0.50%, the refractive index increase due to the updopant cannot be offset by the refractive index reduction due to only fluorine, and as a result, the inner cladding relative to the outer cladding 13 The absolute value of the relative refractive index difference Δ2 of 12 cannot be 0.10% or less. Therefore, the cutoff wavelength may be longer than the desired value, or the bending loss may be greater than the desired value.

(Modification)
The optical fiber 1 functions as a polarization maintaining fiber by adding the stress imparting portions 14a and 14b to the outer cladding 13.

FIG. 7 is a cross-sectional view showing a first configuration example of the optical fiber 1 functioning as a polarization maintaining fiber. In FIG. 7, the refractive index distribution along the line AA (lower side of the sectional view) and the refractive index distribution along the line BB (upper side of the sectional view) are also shown.

The stress applying portions 14 a to 14 b are structures embedded in the outer cladding 13 and having a circular cross section. The refractive index n4 of the stress applying portions 14a to 14b is lower than the refractive index n3 of the outer cladding 13.

In each cross section of the optical fiber 1, the stress applying portions 14a to 14b are arranged symmetrically with respect to the core 11 and the outer edges of the stress applying portions 14a to 14b are in contact with the outer edge of the inner cladding 12, respectively.

FIG. 8 is a cross-sectional view showing a second configuration example of the optical fiber 1 functioning as a polarization maintaining fiber. In FIG. 8, the refractive index distribution along the line AA (lower side of the sectional view) and the refractive index distribution along the line BB (right side of the sectional view) are also shown.

The stress applying portions 14 a to 14 b are structures embedded in the outer cladding 13 and having a circular cross section. The refractive index n4 of the stress applying portions 14a to 14b is lower than the refractive index n3 of the outer cladding 13.

In each cross section of the optical fiber 1, the stress imparting portions 14a to 14b are arranged symmetrically with respect to the core 11 and the outer edges of the stress imparting portions 14a to 14b are separated from the outer edge of the inner cladding 12, respectively. ..

When the structure in which the stress applying portion and the inner clad overlap each other is adopted, the polarization maintaining fiber is manufactured by drawing a preform having a cross section shown in FIG. The relationship of ηoc>ηic>ηsap is established among the viscosity ηoc of the outer clad, the viscosity ηic of the inner clad, and the viscosity ηsap of the stress applying part. Therefore, when drawing, the outer clad, the inner clad and the stress applying part are in this order. It will cure. At this time, the inner clad whose cross section was non-circular in the base material was deformed by the surface tension so that the cross section was circularized, and the core whose cross section was circular in the base material was It transforms to become non-circular. As a result, the cross section of the obtained polarization-maintaining fiber is as shown in FIG. In both the first configuration example shown in FIG. 7 and the second configuration example shown in FIG. 8, the stress applying portions 14 a to 14 b do not overlap the inner cladding 12. Therefore, it is possible to suppress the non-circularization of the core 11 that may occur during drawing. Therefore, it is possible to realize the optical fiber 1 in which the core 11 has a low non-circularity, that is, the core 11 has a high circularity, as compared with the case where the stress applying portion and the inner cladding overlap each other.

(Application example)
The optical fiber 1 according to this embodiment can be suitably used as a bridge fiber interposed between a semiconductor optical waveguide and a CSMF (Conventional Single Mode Fiber).

That is, as shown in FIG. 10, the bridge fiber 102 configured by the optical fiber 1 according to the present embodiment, the semiconductor optical waveguide 101 butt-connected to one end of the bridge fiber 102, and the other end of the bridge fiber 102 fused to each other. The optical device 100 with low loss can be configured with the CSMF 103 that is connected and connected. This is because, when the bridge fiber 102 and the CSMF 103 are fusion-spliced, the core of the bridge fiber 102 is sufficiently expanded in the vicinity of the fusion point, so that the splice loss at the fusion point between the bridge fiber 102 and the CSMF 103 is sufficient. This is because it can be suppressed.

As the semiconductor optical waveguide 101, for example, a silicon waveguide formed with an SSC (Spot Size Converter) is suitable. Further, as the CSMF 103, for example, a single mode fiber having a mode field diameter of about 10 μm at a wavelength of 1550 nm is suitable. G. in ITU-T. 652 or G.I. The optical fiber classified as 657 is an example of such a single mode fiber.

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

1 Optical Fiber 11 Core 12 Inner Clad 13 Outer Clad 14a Stress Applying Part 14b Stress Applying Part

Claims (9)

G. in ITU-T. 652 or G.I. An optical fiber for fusion splicing with a CSMF (Conventional Single Mode Fiber) classified as 657,
A core made of quartz glass with an updopant added,
An inner clad covering the side surface of the core, wherein the inner clad is made of quartz glass to which both up-dopant and down-dopant are added,
An outer clad covering the outer surface of the inner clad, comprising an outer clad made of quartz glass,
The refractive index of the inner cladding is substantially equal to the refractive index of the outer cladding,
The concentration of the updopant in the inner cladding is determined so that the rate of increase in the refractive index due to the updopant is 0.25% or more and 0.5% or less,
The concentration of the up dopants in the core is defined as the mode field diameter at a wavelength of 1550nm is 3.5μm or 6.5μm or less,
The updopant added to the core is germanium,
The down dopant added to the inner cladding is fluorine,
The updopant added to the inner cladding is both phosphorus and germanium,
An optical fiber characterized in that. The concentration of the down dopant in the inner clad is determined so that the absolute value of the relative refractive index difference of the inner clad with respect to the outer clad is 0.1% or less.
The optical fiber according to claim 1, wherein: Above the inner cladding, as the downdopants further optical fiber according to claim 1 or 2, characterized in that boron is added. Further comprising a pair of stress applying portions symmetrically arranged with respect to the core,
The optical fiber according to any one of claim 1 to 3, characterized in that. The pair of stress applying portions are arranged such that an outer edge thereof is in contact with an outer edge of the inner clad, or an outer edge thereof is separated from an outer edge of the inner clad,
The optical fiber according to claim 4 , wherein: At least one end portion of the optical fiber during or after the heat treatment, a first region having a relative refractive index difference value relatively lower than that of the inner cladding is present in at least the outer cladding, a second region having a value of relatively high relative refractive index difference than that of the first region is present outside the said first region, any one of claim 1 to 5, characterized in that The optical fiber described in. The at least one region where the concentration of the downdopant is positive is present inside the core at least at one end of the optical fiber during or after the heat treatment. any optical fiber according to one of 1-6. At least one end of the optical fiber after heat treatment, the concentration of the downdopant in the entire inside of the core becomes positive,
The optical fiber according to claim 7 , wherein: An optical device comprising: the optical fiber according to any one of claims 1 to 8 ; and the CSMF fusion-spliced to the other end of the optical fiber.

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