JP6986904B2 - Optical fiber, optical device, and manufacturing method of optical device - Google Patents

Description

The present invention relates to an optical fiber comprising a core and a cladding, an optical device comprising such an optical fiber, and a method of manufacturing such an optical device.

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

A general-purpose 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, while the mode field diameter of the semiconductor optical waveguide is about 1 μm, the mode field diameter of the general-purpose single mode fiber is about 10 μm. Therefore, when a general-purpose single-mode fiber is butt-connected to a semiconductor optical waveguide, the connection loss due to the difference in mode field diameter becomes too large to withstand practical use.

Therefore, a method has been proposed in which a spot size converter is built in a semiconductor optical waveguide and a general-purpose single-mode fiber is connected to the spot size converter (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 the general-purpose single-mode fiber by using the spot size converter, there is a problem that the loss in the spot size converter becomes large. Therefore, one end of a bridge fiber having a mode field diameter of about 4 μm is abutted and connected to a semiconductor optical waveguide whose mode field diameter is expanded to about 4 μm by using a spot size converter, and a mode field is connected to the other end of the bridge fiber. It is being studied to fuse and connect general-purpose single-mode fibers having a diameter of 10 μm.

As such a bridge fiber, a TEC (Thermally Diffused Expanded Core) fiber, that is, an optical fiber whose core diameter can be expanded by heating is used (see Patent Document 2 and Non-Patent Document 1). Then, since the core of the bridge fiber expands at the time of fusion with the CSMF or by heating thereafter, the mismatch of the mode field diameter between the bridge fiber and the CSMF is reduced, and as a result, the fusion of the bridge fiber and the CSMF is reduced. This is because the connection loss at the landing point can be kept small.

The expansion of the core due to thermal diffusion is caused by the updopant (additive for increasing the refractive index of quartz glass) added to form the core diffuses to the surroundings by heating. When the updopant added to form the core is germanium (Ge), it is known that the expansion of the core can be promoted by co-adding germanium, phosphorus (P), and fluorine (F) to the surroundings. (Non-Patent Document 1).

Patent No. 5900484 (issued on April 6, 2016) Patent No. 3993198 (issued on October 17, 2007)

K. Hirakawa, T. Oda, K. Ichii, and K. Aikawa, "Small Core Diameter TEC Fiber for Silicon Optical Waveguides", IEICE Technical Report, October 20, 2016, vol.116, no. 273, pp.37-41

Regarding the spatial coupling between the optical fiber and the semiconductor optical waveguide, the difference in the mode field diameter between the two has been considered in the past, but the difference in the electric field distribution between the two has not been considered. Therefore, in the conventional optical fiber, there is room for improving the coupling efficiency with the semiconductor optical waveguide.

The present invention has been made in view of the above problems, and an object of the present invention is to realize an optical fiber having improved coupling efficiency with a semiconductor optical waveguide without unnecessarily increasing bending loss.

The optical fiber according to the present invention includes a core and a clad covering the side surface of the core, the diameter R of the core is 4.7 μm or less, and the specific refractive index difference Δ of the core with respect to the clad is 0.7. % Or more, the sign of the rate of change dω / dR of the mode field diameter ω with respect to the diameter R of the core is negative, and the bending loss at a wavelength of 1550 nm when the bending radius is 7.5 mm is 0.1 dB / turn or less. It is characterized by being.

In the optical fiber according to the present invention, the diameter R of the core is 4.2 μm or less, the specific refractive index difference Δ of the core with respect to the clad is 0.9% or more, and the mode field with respect to the diameter R of the core. It is preferable that the sign of the rate of change dω / dR of the diameter ω is negative and the bending loss at a wavelength of 1550 nm when the bending radius is 5 mm is 0.1 dB / turn or less.

In the optical fiber according to the present invention, the core is made of quartz glass to which an updopant is added, and the clad includes an inner clad covering the side surface of the core and an outer clad covering the side surface of the inner clad. , And the inner clad is made of quartz glass to which both up-dopant and down-dopant are added, and the absolute value | Δ2 | of the specific refractive index difference Δ2 of the inner clad with respect to the outer clad is 0. It is preferably 1% or less.

The optical device according to the present invention includes the above optical fiber and a semiconductor optical waveguide spatially coupled to the optical fiber.

The optical device according to the present invention includes the above-mentioned optical fiber and a general-purpose single-mode fiber fused and connected to the optical fiber.

The manufacturing method according to the present invention is a method for manufacturing an optical device including the above-mentioned optical fiber and a semiconductor optical waveguide spatially coupled to the optical fiber, wherein the above-mentioned optical fiber is used as a first optical fiber. The optical fiber having a positive sign of the rate of change dω / dR obtained by heating the above optical fiber is used as the second optical fiber, and the electric field distribution of the first optical fiber is higher than that of the semiconductor optical fiber. The step of determining whether the electric field distribution of the second optical fiber is closer to the electric field distribution of the waveguide and the electric field distribution of the second optical fiber is closer to the electric field distribution of the semiconductor optical waveguide is included, and the electric field distribution of the first optical fiber is the same as described above. When it is determined that the electric field distribution is close to that of the semiconductor optical waveguide, the optical device is manufactured by spatially coupling the first optical fiber to the semiconductor optical waveguide, and the electric field distribution of the second optical fiber is larger than that of the second optical fiber. When it is determined that the electric field distribution is close to that of the semiconductor optical waveguide, the optical device is manufactured by spatially coupling the second optical fiber to the semiconductor optical waveguide.

According to the present invention, it is possible to realize an optical fiber having improved coupling efficiency with a semiconductor optical waveguide without unnecessarily increasing bending loss.

It is a figure which shows the structure and the refractive index distribution of the optical fiber which concerns on one Embodiment of this invention. The upper part is a cross-sectional view showing the structure of the optical fiber, and the lower part is a graph showing the refractive index distribution of the optical fiber. It is a graph which shows the relationship between the core diameter R and the mode field diameter ω in the optical fiber shown in FIG. The graph shown in (a) is obtained when the core Δ is 0.3%, and the graph shown in (b) is obtained when the core Δ is 1.0%. Yes, the graph shown in (c) was obtained when the core Δ was 2.0%, and the graph shown in (d) was obtained when the core Δ was 2.5%. It is a thing. It is a graph which shows the electric field distribution of the optical fiber shown in FIG. The graph shown by the solid line shows the electric field distribution of the optical fiber having the mode field diameter ω of 4.3 μm and the sign of the rate of change dω / dR is negative, and the graph shown by the dotted line shows the mode field. Among the optical fibers having a diameter ω of 4.3 μm, the electric field distribution of the optical fiber having a positive sign of the rate of change dω / dR is shown. For the optical fiber shown in FIG. 1, the combination of the core diameter R and the core Δ at which the rate of change dω / dr becomes 0 is plotted with a solid line, and the bending loss at a wavelength of 1550 nm is 0 when the bending radius is 7.5 mm. It is a graph which plotted the combination of the core diameter R which becomes 1dB / turn and the core Δ by the dotted line. For the optical fiber shown in FIG. 1, the combination of the core diameter R and the core Δ at which the rate of change dω / dr becomes 0 is plotted with a solid line, and the bending loss at a wavelength of 1550 nm is 0.1 dB / when the bending radius is 5 mm. It is a graph which plotted the combination of the core diameter R which becomes a turn and the core Δ by a dotted line. (A) is a graph showing the electric field distribution of the optical fiber according to the first embodiment and in which the sign of the rate of change dω / dR is negative, and (b) is the light according to the embodiment. It is a graph which shows the electric field distribution of the optical fiber which is a fiber, and the sign of the rate of change dω / dR is positive. (A) is a graph showing the electric field distribution of the optical fiber according to the second embodiment, and the sign of the rate of change dω / dR is negative, and (b) is the light according to the embodiment. It is a graph which shows the electric field distribution of the optical fiber which is a fiber, and the sign of the rate of change dω / dR is positive. (A) is a side view showing the first example of the optical device provided with the optical fiber shown in FIG. 1, and (b) is a second example of the optical device provided with the optical fiber shown in FIG. It is a side view which shows.

[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.

The optical fiber 1 is a columnar structure mainly composed of quartz glass, and as shown in the upper part of FIG. 1, (1) a core 11 having a circular (radius r1) cross section and (2) a core. A clad 12 having an annular (inner peripheral radius r1, outer peripheral radius r3) -shaped cross section covering the side surface of the ring is provided. The clad 12 has an inner clad 12a having a cross section in the shape of an annulus (inner peripheral radius r1, outer peripheral radius r2) covering the side surface of the core (2a), and an annular ring (inner) covering the side surface of the inner clad 12a (2b). The outer clad 12b having a cross section having a peripheral radius r2 and an outer peripheral radius r3) is included. The optical fiber 1 may further include (4) a protective coating layer (not shown) having an annular cross section that covers the outer surface of the outer clad 12b.

The core 11 is made of quartz glass to which germanium (Ge), which is an updopant, is added. The refractive index distribution of the core 11 is the uniform distribution shown in the lower part of FIG. 1, or is approximated by the uniform distribution shown in the lower part of FIG. As shown in the lower part of FIG. 1, the refractive index n1 of the core 11 is higher than the refractive index n2b of the outer clad 12b described later. When the refractive index distribution of the core 11 is not a perfect uniform distribution, the refractive index n1 of the core 11 refers to the average value of the refractive index of the core 11. From the refractive index n1 of the core 11 and the refractive index n2b of the outer clad 12b, the amount Δ1 defined by Δ1 = {(n1-n2b) / n1} × 100 is the difference in the specific refractive index of the core 11 with respect to the outer clad 12b. Call.

The concentration of the updopant added to the core 11 is preferably set so that the specific refractive index difference Δ1 is 0.7% or more and 4% or less, and the specific refractive index difference Δ1 is 0.9% or more and 4%. It is more preferable that the settings are as follows.

If the specific refractive index difference Δ1 becomes too small, there may be a problem that the bending loss exceeds the upper limit of the allowable range. For example, if the difference in the specific refractive index is made smaller than 0.9%, the problem that the bending loss at a wavelength of 1550 nm when the bending radius is 5 mm exceeds 0.1 dB / turn tends to occur. Further, when the difference in the specific refractive index is made smaller than 0.7%, the problem that the bending loss per turn when the bending radius is 7.5 mm exceeds 0.1 dB at a wavelength of 1550 nm tends to occur. If the specific refractive index difference Δ1 is set as described above, such a problem is less likely to occur.

On the contrary, if the specific refractive index difference Δ1 is set too large, the stress generated between the core and the clad becomes large when the base material of the optical fiber 1 is manufactured, and as a result, the base material is processed using a lathe or the like. In doing so, there may be a problem that the base metal is easily cracked. If the specific refractive index difference Δ1 is set as described above, such a problem is less likely to occur. Further, from the viewpoint of the efficiency of adding GeO2 to SiO2, the specific refractive index difference Δ1 is preferably 4% or less.

In the present embodiment, a configuration is adopted in which germanium is added to the core 11 as an updopant, but the present invention is not limited to this. For example, a configuration in which phosphorus (P) is added to the core 11 as an updopant 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 12a is made of quartz glass to which fluorine (F) as a down dopant and germanium (Ge) and phosphorus (P) as up dopants are added. The updopant added to the inner clad 12a is for promoting thermal diffusion of the updopant added to the core 11. The down dopant added to the inner clad 12a is for offsetting the decrease in the refractive index due to the up dopant added to the inner clad 12a. The refractive index distribution of the inner clad 12a is the uniform distribution shown in the lower part of FIG. 1, or is approximated by the uniform distribution shown in the lower part of FIG. As shown in the lower part of FIG. 1, the refractive index n2a of the inner clad 12a is substantially the same as the refractive index n2b of the outer clad 12b described later. When the refractive index distribution of the inner clad 12a is not a perfect uniform distribution, the refractive index n2a of the inner clad 12a refers to the average value of the refractive indexes of the inner clad 12a. From the refractive index n2a of the inner clad 12a and the refractive index n2b of the outer clad 12b, the amount Δ2 defined by Δ2 = {(n2a-n2b) / n2a} × 100 is the specific refractive index of the inner clad 12a with respect to the outer clad 12b. Called the difference.

The concentrations of the down dopant and the up dopant added to the inner clad 12a are preferably set so that the absolute value | Δ2 | of the specific refractive index difference Δ2 is 0.1% or less.

If the absolute value | Δ2 | of the specific refractive index difference Δ2 becomes too large, the refractive index difference between the core 11 and the inner clad 12a has a non-negligible effect on the optical characteristics of the optical fiber 1. For example, when the specific refractive index difference Δ2 is made smaller than −0.1, the confinement of light in the core 11 by the inner clad 12a becomes stronger, and the cutoff wavelength exceeds the upper limit of the allowable range. Further, when the specific refractive index difference Δ2 is made larger than 0.1, the confinement of light in the core 11 by the inner clad 12a is weakened, which may cause a problem that the bending loss exceeds the upper limit of the allowable range. If the specific refractive index difference Δ2 is set as described above, such a problem is less likely to occur.

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

The outer clad 12b is made of quartz glass to which a dopant other than chlorine (Cl) is not intentionally added. The refractive index distribution of the outer clad 12b is the uniform distribution shown in the lower part of FIG. 1, or is approximated by the uniform distribution shown in the lower part of FIG. Since neither the up dopant nor the down dopant is added to the quartz glass constituting the outer clad 12b, the refractive index n2b of the outer clad 12b is substantially the same as the refractive index 1.46 of the pure quartz glass. When the refractive index distribution of the outer clad 12b is not a perfect uniform distribution, the refractive index n2b of the outer clad 12b refers to the average value of the refractive indexes of the outer clad 12b.

As described above, in the optical fiber 1, the inner cladding 12a contains a down dopant. This down-dopant has the action of (1) reducing the refractive index n1 of the core 11 by thermally diffusing itself into the core 11 and increasing the refractive index n2a of the inner clad 12a, and (2) the core 11. By promoting the thermal diffusion of the added updopant to the inner clad 12a, it has the effect of increasing the refractive index n2a of the inner clad 12a. Generally speaking, this down dopant has an effect of reducing the refractive index difference n1-n2a between the core 11 and the inner clad 12a. Therefore, the optical fiber 1 functions as a TEC (Thermally-Diffused Expanded Core) fiber whose core diameter can be expanded by heating.

[Preferable combination of core diameter and core Δ]
The relationship between the core diameter and the mode field diameter in the optical fiber 1 will be described with reference to FIGS. 2 and 3. The core diameter of the optical fiber 1 refers to the diameter of the core 11 (twice the diameter of r1 in FIG. 1). In the following, the core diameter of the optical fiber 1 is represented by the symbol R. Further, a mode field diameter of the optical fiber 1 with respect intensity distribution of the fundamental mode in the cross section of the optical fiber 1, 1 / e ² times the light intensity peak intensity of a region to be (0.135 times the peak intensity) diameter Refers to. In the following, the mode field diameter of the optical fiber 1 is represented by the symbol ω. The mode field diameter ω is larger than the core diameter R. This is because a part of the light guided through the core 11 seeps into the inner clad 12a.

FIG. 2 is a graph showing the relationship between the core diameter R and the mode field diameter ω in the optical fiber 1. The graph shown in FIG. 2A is obtained when the core Δ is 0.3%, and the graph shown in FIG. 2B is when the core Δ is 1.0%. The graph shown in FIG. 2 (c) is obtained when the core Δ is 2.0%, and the graph shown in FIG. 2 (d) is the core Δ. It was obtained when the value was 2.5%. The core Δ refers to the specific refractive index difference defined by Δ = {(n1-n2) / n1} × 100, where the average refractive index of the clad 12 is n2.

By referring to FIG. 2, the following can be confirmed. That is, as the core diameter R is increased, the mode field diameter ω changes from decreasing to increasing at a certain core radius R0. In other words, as the core diameter R is increased, the sign of the rate of change dω / dR changes from negative to positive at a certain core radius R0. For example, when the core Δ is 0.3%, the sign of the rate of change dω / dR changes from negative to positive with the core radius R0 = 7.9 μm as a boundary.

Therefore, in the equation ω (R) = ω0, the first solution R = Ra and the second solution R = Rb exist (Ra <Rb). In other words, as the optical fiber 1 having the mode field diameter ω of ω0, there is a first optical fiber 1 having a core diameter R of Ra and a second optical fiber 1 having a core diameter R of Rb. For example, when the core Δ is 0.3%, as the optical fiber 1 having the mode field diameter ω of 4.3 μm, the first optical fiber 1 having the core diameter R of 2.5 μm and the core diameter R of 3 There is a second optical fiber 1 having a diameter of 0.7 μm.

Of the two optical fibers 1 having the same mode field diameter ω, the first optical fiber 1 having a core diameter R of Ra is characterized by having a negative sign of the rate of change dω / dR of the mode field diameter. Be done. Further, of the two optical fibers 1 having the same mode field diameter ω, the second optical fiber 1 having a core diameter R of Rb has a positive sign of the rate of change dω / dR of the mode field diameter. Characterized.

FIG. 3 is a graph showing the electric field distributions of the two optical fibers 1 having the mode field diameter ω of 4.3 μm. The graph shown in FIG. 3 is obtained when the core Δ is 0.3%. In FIG. 3, the graph represented by the solid line shows the electric field distribution of the optical fiber 1 in which the sign of the rate of change dω / dR is negative, and the graph represented by the dotted line is the light in which the sign of the rate of change dω / dR is positive. The electric field distribution of the fiber 1 is shown. In FIG. 3, the vertical axis is the standardized electric field strength obtained by normalizing the electric field strength with the peak strength as a reference.

By referring to FIG. 3, the following can be confirmed. That is, the shape of the electric field distribution is different between the two optical fibers 1 having the same mode field diameter ω. Specifically, in the optical fiber 1 in which the sign of the rate of change dω / dR is negative, the confinement of light in the core 11 is weak and the amount of light seeping into the inner clad 12a is large, whereas the rate of change dω In the optical fiber 1 in which the sign of / dR is positive, the light is strongly confined in the core 11 and the amount of light seeping into the inner clad 12a is small.

Here, consider connecting the optical fiber 1 to another optical waveguide. Similar to the optical fiber 1 in which the sign of the rate of change dω / dR is negative, the optical waveguide has a weak confinement of light in the core and a large amount of light seeping into the clad, and a sign of the rate of change dω / dR. Similar to the positive optical fiber 1, some optical fibers have strong confinement of light in the core and a small amount of light seeping into the clad. The optical fiber 1 having a negative sign of the rate of change dω / dR when connected to another optical waveguide has the following merits. (1) When connected to the former optical waveguide, higher coupling efficiency than that of the optical fiber 1 in which the sign of the rate of change dω / dR is positive can be obtained. (2) By heating until the core diameter R changes from Ra to Rb when connecting to the latter optical waveguide, the coupling efficiency equivalent to that of the optical fiber 1 having a positive sign of the rate of change dω / dR can be obtained. can get.

However, in order to make the sign of the rate of change dω / dR negative, it is necessary to reduce the core diameter R and the core Δ. Therefore, the small core optical fiber 1 has a demerit that the bending loss tends to be large. Therefore, when designing the small core optical fiber 1, it is necessary to set the core diameter R and the core Δ so that the bending loss does not exceed the upper limit of the allowable range.

Hereinafter, a combination of a core diameter R and a core Δ in which the sign of the rate of change dω / dR becomes negative while satisfying the low bending loss condition will be examined. The following two conditions are considered as low bending loss conditions.

Low bending loss condition 1: The bending loss at a wavelength of 1550 nm when the bending radius is 7.5 mm is 0.1 dB / turn or less.

Low bending loss condition 2: The bending loss at a wavelength of 1550 nm when the bending radius is 5 mm is 0.1 dB / turn or less.

In FIG. 4, the combination of the core diameter R and the core Δ at which the rate of change dω / dr becomes 0 is plotted with a solid line, and the bending loss at a wavelength of 1550 nm is 0.1 dB / turn when the bending radius is 7.5 mm. It is a graph which plotted the combination of the core diameter R and the core Δ by the dotted line. In FIG. 4, the region where the sign of the rate of change dω / dr is negative is the region located at the lower left of the graph shown by the solid line. On the other hand, the region satisfying the low bending loss condition 1 is a region located on the upper right side of the graph shown by the dotted line.

By referring to FIG. 4, the following can be confirmed. That is, if the core diameter R is 4.7 μm or less and the core Δ is 0.7 or more, a common portion (hatched region in FIG. 4) exists in the above-mentioned two regions. In other words, if the core diameter R is 4.7 μm or less and the core Δ is 0.7 or more, the sign of the rate of change dω / dr can be made negative and the low bending loss condition 2 can be satisfied.

FIG. 5 plots the combination of the core diameter R and the core Δ at which the rate of change dω / dr is 0 with a solid line, and the core has a bending loss of 0.1 dB / turn at a wavelength of 1550 nm when the bending radius is 5 mm. It is a graph which plotted the combination of the diameter R and the core Δ by the dotted line. In FIG. 5, the region where the sign of the rate of change dω / dr is negative is the region located at the lower left of the graph shown by the solid line. On the other hand, the region satisfying the low bending loss condition 2 is a region located on the upper right side of the graph shown by the dotted line.

By referring to FIG. 5, the following can be confirmed. That is, if the core diameter R is 4.2 μm or less and the core Δ is 0.9 or more, there is a common portion (hatched region in FIG. 4) in the above-mentioned two regions. In other words, if the core diameter R is 4.2 μm or less and the core Δ is 0.9 or more, the sign of the rate of change dω / dr can be made negative and the low bending loss condition 2 can be satisfied.

As described above, in the optical fiber 1 according to the present embodiment, it is preferable that the diameter R of the core 11 core is 4.7 μm or less, and the core Δ of the core 11 with respect to the outer clad 12b is 0.7% or more. .. As a result, the sign of the rate of change dω / dR of the mode field diameter ω with respect to the core diameter R is made negative, and the bending loss at a wavelength of 1550 nm when the bending radius is 7.5 mm is made 0.1 dB / turn or less. be able to.

Further, in the optical fiber 1 according to the present embodiment, it is preferable that the diameter R of the core 11 is 4.2 μm or less, and the core Δ of the core 11 with respect to the outer clad 12b is 0.9% or more. As a result, the sign of the rate of change dω / dR of the mode field diameter ω with respect to the diameter R of the core 11 is made negative, and the bending loss at the wavelength of 1550 nm when the bending radius is 5 mm is 0.1 dB / turn or less. Can be done.

[Example 1]
As an example of the optical fiber 1 according to the present embodiment, the core diameter R (radius r1 × 2 of the core 11) is 2.4 μm, the core Δ is 2.2%, and the inner clad diameter (inner clad 12a). An optical fiber A having an outer peripheral radius r2a × 2) of 16 μm, an outer clad diameter (outer peripheral radius r2b × 2 of the outer clad 12b) of 125 μm, and a negative sign of the rate of change dω / dR was created. In the optical fiber A, the mode field diameter ω at a wavelength of 1550 nm was 4.3 μm. Further, in the optical fiber A, the bending loss at a wavelength of 1550 nm when the bending radius was 7.5 mm was 0.1 dB / turn or less.

Further, by heating the optical fiber A having a negative sign of the rate of change dω / dR until the core diameter R changes from 2.4 μm to 4.0 μm, the sign of the rate of change dω / dR is positive. An optical fiber B was created. Also in the optical fiber B, the mode field diameter ω at a wavelength of 1550 nm was 4.3 μm.

FIG. 6A is a graph showing the electric field distribution of the optical fiber A in which the sign of the rate of change dω / dR is negative, and FIG. 6B is a graph showing the sign of the rate of change dω / dR being positive. It is a graph which shows the electric field distribution of an optical fiber B. In FIG. 6A and FIG. 6B, the electric field distribution of the silicon optical waveguide S having a spot diameter of 4 μm is also shown. In the graph shown in FIG. 6, "Horizontal" indicates the electric field distribution in the direction parallel to the substrate surface of the silicon optical waveguide S, and "Vertical" indicates the electric field distribution in the direction perpendicular to the substrate surface of the silicon optical waveguide S. Is shown.

Comparing (a) of FIG. 6 and (b) of FIG. 6, the electric field distribution of the optical fiber A in which the sign of the rate of change dω / dR is negative has a positive sign of the rate of change dω / dR. It can be seen that the electric field distribution of the silicon optical waveguide S is closer than the electric field distribution of the optical fiber B.

When the coupling efficiency η between the optical fiber A having a negative sign of the rate of change dω / dR and the silicon optical waveguide S was calculated according to the following equation (1), η = 95%. Further, when the coupling efficiency η between the optical fiber B having a positive sign of the rate of change dω / dR and the silicon optical waveguide S was calculated according to the following equation (1), η = 88%. From this, the coupling loss of the optical device in which the optical fiber A and the silicon optical waveguide S are spatially coupled is 0.3 dB smaller than the coupling loss of the optical device in which the optical fiber B and the silicon optical waveguide S are spatially coupled. It turned out to be.

[Example 2]
As an example of the optical fiber 1 according to the present embodiment, the core diameter R (radius r1 × 2 of the core 11) is 3.9 μm, the core Δ is 1.1%, and the inner clad diameter (inner clad 12a). An optical fiber C having an outer peripheral radius r2a × 2) of 28 μm, an outer clad diameter (outer peripheral radius r2b × 2 of the outer clad 12b) of 125 μm, and a negative sign of the rate of change dω / dR was created. In the optical fiber C, the mode field diameter ω at a wavelength of 1550 nm was 6.1 μm.

Further, by heating the optical fiber C having a negative sign of change rate dω / dR, an optical fiber D having a positive sign of change rate dω / dR was created. Also in the optical fiber D, the mode field diameter ω at a wavelength of 1550 nm was 6.1 μm. Further, in the optical fiber D, the bending loss at a wavelength of 1550 nm when the bending radius was 7.5 mm was 0.1 dB / turn or less.

FIG. 7A is a graph showing the electric field distribution of the optical fiber C in which the sign of the rate of change dω / dR is negative, and FIG. 7B is a graph showing the sign of the rate of change dω / dR being positive. It is a graph which shows the electric field distribution of an optical fiber D. In FIG. 7A and FIG. 7B, the electric field distribution of the silicon optical waveguide T having a spot diameter of 6 μm is also shown. In the graph shown in FIG. 6, "Horizontal" indicates the electric field distribution in the direction parallel to the substrate surface of the silicon optical waveguide T, and "Vertical" indicates the electric field distribution in the direction perpendicular to the substrate surface of the silicon optical waveguide T. Is shown.

Comparing (a) of FIG. 7 and (b) of FIG. 7, the electric field distribution of the optical fiber D in which the sign of the rate of change dω / dR is positive has a negative sign of the rate of change dω / dR. It can be seen that the electric field distribution of the silicon optical waveguide T is closer than the electric field distribution of the optical fiber C.

When the coupling efficiency η between the optical fiber D having a positive sign of the rate of change dω / dR and the silicon optical waveguide T was calculated according to the above equation (1), η = 91%. Further, when the coupling efficiency η between the optical fiber C having a negative sign of the rate of change dω / dR and the silicon optical waveguide T was calculated according to the following equation (1), η = 84%. From this, the coupling loss of the optical device in which the optical fiber D and the silicon optical waveguide T are spatially coupled is 0.4 dB smaller than the coupling loss of the optical device in which the optical fiber C and the silicon optical waveguide T are spatially coupled. It turned out to be.

[Optical device and its manufacturing method]
The optical fiber 1 according to the present embodiment can form an optical device together with other optical elements. An example of the configuration of such an optical device is shown in FIG. The optical device 10A shown in FIG. 8A is an optical device including the optical fiber 1 according to the present embodiment and the semiconductor optical waveguide 2 spatially coupled to the optical fiber 1. Examples of the semiconductor optical waveguide 2 include a silicon optical waveguide having a spot size converter. The optical device 10B shown in FIG. 8B is an optical device including the optical fiber 1 according to the present embodiment and a general-purpose single-mode fiber 3 fused and connected to the optical fiber 1.

The optical device 10A shown in FIG. 8A can be efficiently mass-produced by manufacturing using a manufacturing method including the following steps.

Step 1: Create a plurality of semiconductor optical waveguides 2.

Step 2: A plurality of optical fibers 1 having a negative sign of the rate of change dω / dR are produced.

Step 3: The optical fiber 1 selected from the plurality of optical fibers 1 created in step 2 is used as the first sample, and the electric field distribution of the first sample is measured.

Step 4: A second sample is an optical fiber 1 having a positive sign of change rate dω / dR, which is obtained by heating another optical fiber 1 selected from the plurality of optical fibers 1 created in step 2. Then, the electric field distribution of the second sample is measured.

After performing the above steps 1 to 4, the following steps are carried out for each of the plurality of semiconductor optical waveguides 2 created in the above step 1.

Step 5: The electric field distribution of the semiconductor optical waveguide 2 is measured.

Step 6: Of the electric field distribution of the first sample measured in step 3 and the electric field distribution of the second sample measured in step 4, which electric field distribution is measured in step 5 is the semiconductor optical waveguide 2. Judge whether it is close to the electric field distribution of. The perspective of the two electric field distributions can be determined, for example, based on the magnitude of the square error of these two electric field distributions.

When it is determined in step 6 that the electric field distribution of the first sample is closer to the electric field distribution of the semiconductor optical waveguide 2, the optical device 10A is manufactured by carrying out the following step 7. In this case, the following steps 8 to 9 are not carried out. On the other hand, when it is determined in step 6 that the electric field distribution of the second sample is closer to the electric field distribution of the semiconductor optical waveguide 2, the optical device 10A is manufactured by carrying out the following steps 8 to 9. In this case, the following step 7 is not performed.

Step 7: The semiconductor optical waveguide 2 is spatially coupled to the optical fiber 1 having a negative sign of the rate of change dω / dR created in step 1. Before spatially coupling the semiconductor optical waveguide 2 to the optical fiber 1, a step of smoothing the end face of the optical fiber 1 may be performed by using a fiber cleaver.

Step 8: By heating the optical fiber 1 manufactured in step 1, the optical fiber 1 having a positive sign of the rate of change dω / dR is produced. For heating the optical fiber 1, for example, an arc discharge type fusion splicer may be used. Further, it is preferable that the power for heating the optical fiber 1 is lower than the power for fusion-bonding the general-purpose single-mode fibers. If the power for heating the optical fiber 1 is too large, the optical fiber 1 may have an unfavorable outer shape change, or it may be difficult to change the core diameter R of the optical fiber 1 to a desired value. Because there is.

Step 9: The semiconductor optical waveguide 2 is spatially coupled to the optical fiber 1 having a positive sign of the rate of change dω / dR created in step 8. Before spatially coupling the semiconductor optical waveguide 2 to the optical fiber 1, a step of smoothing the end face of the optical fiber 1 may be performed by using a fiber cleaver.

By using the above manufacturing method, it is possible to efficiently mass-produce the optical device 10A having high coupling efficiency even when the electric field distribution of the semiconductor optical waveguide 2 produced in step 1 varies.

The optical device 10B shown in FIG. 8B can be manufactured by fusion-bonding the optical fiber 1 and the general-purpose single-mode fiber 3. Since the core expansion of the optical fiber 1 occurs due to the heat generated when the optical fiber 1 and the general-purpose single-mode fiber 3 are fused and connected, the mode field diameter mismatch that may occur at the boundary between the optical fiber 1 and the general-purpose single-mode fiber 3 occurs. Is reduced.

[Additional notes]
The present invention is not limited to the above-described embodiment, and various modifications can be made within the scope of the claims, and the present invention also relates to an embodiment obtained by appropriately combining the technical means disclosed in the present specification. Included in the technical scope of the invention.

1 Optical fiber 11 core 12 clad 12a inner clad 12b outer clad

Claims (7)

With a core and a clad covering the sides of the core,
The diameter R of the core is 4.7 μm or less, and the core has a diameter R of 4.7 μm or less.
The specific refractive index difference Δ of the core with respect to the cladding is 0.7% or more.
The diameter R of the core is determined so that when the diameter R of the core changes due to heating , the sign of the rate of change dω / dR of the mode field diameter ω with respect to the diameter R of the core becomes negative.
The bending loss at a wavelength of 1550 nm when the bending radius is 7.5 mm is 0.1 dB / turn or less.
An optical fiber characterized by that. The specific refractive index difference Δ of the core with respect to the cladding is less than 2.0%.
The optical fiber according to claim 1. The diameter R of the core is 4.2 μm or less, and the core has a diameter R of 4.2 μm or less.
The specific refractive index difference Δ of the core with respect to the cladding is 0.9% or more.
The sign of the rate of change dω / dR of the mode field diameter ω with respect to the diameter R of the core is negative.
The bending loss at a wavelength of 1550 nm when the bending radius is 5 mm is 0.1 dB / turn or less.
The optical fiber according to claim 1 or 2. The core is made of quartz glass with an updopant added.
The clad includes an inner clad covering the side surface of the core and an outer clad covering the side surface of the inner clad.
The inner clad is made of quartz glass to which both up-dopant and down-dopant are added.
The absolute value | Δ2 | of the specific refractive index difference Δ2 of the inner clad with respect to the outer clad is 0.1% or less.
The optical fiber according to any one of claims 1 to 3. An optical device comprising the optical fiber according to claim 4 and a semiconductor optical waveguide spatially coupled to the optical fiber. An optical device comprising the optical fiber according to claim 4 and a general-purpose single-mode fiber fused and connected to the optical fiber. A method for manufacturing an optical device including an optical fiber and a semiconductor optical waveguide spatially coupled to the optical fiber.
The optical fiber according to claim 4 is used as the first optical fiber, and the optical fiber having a positive sign of the rate of change dω / dR obtained by heating the optical fiber according to claim 4 is the second optical fiber. As an optical fiber, whether the electric field distribution of the first optical fiber is closer to the electric field distribution of the semiconductor optical waveguide, or the electric field distribution of the second optical fiber is closer to the electric field distribution of the semiconductor optical waveguide. Including the judgment step
When it is determined that the electric field distribution of the first optical fiber is closer to the electric field distribution of the semiconductor optical waveguide, the optical device is manufactured by spatially coupling the first optical fiber to the semiconductor optical waveguide. When it is determined that the electric field distribution of the second optical fiber is closer to the electric field distribution of the semiconductor optical waveguide, the optical device is manufactured by spatially coupling the second optical fiber to the semiconductor optical waveguide. A method of manufacturing an optical device, characterized in that.

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