JP2007310187A - Mode field conversion fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mode field conversion fiber which can easily be manufactured and can highly efficiently and simly convert a mode field. <P>SOLUTION: The mode field conversion fiber 10 includes a core region 11 for propagating light, a clad region 12 provided in a manner surrounding the core region 11, a plurality of vacancies 13 formed in the clad region 12 in a manner surrounding the core region 11 with a prescribed interval, and a medium 14 that is filled only on one end of the vacancies 13 and that has an arbitrary refractive index n3, wherein the interior of the other end of the vacancies 13 is air. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a mode field conversion fiber used for optical connection in an optical transmission system or the like.

  In recent years, various optical fibers according to applications have been proposed. These optical fibers generally have different mode fields. When optical fibers of different mode fields are connected, loss occurs due to mode field mismatch. This loss increases as the mode field of the optical fiber to be connected differs greatly. For this reason, when optical fibers of different mode fields are connected, mode field mismatch loss can be reduced by applying a spatial coupling system using a lens or a TEC (Thermally Expanded Core) fiber using a tapered structure. Is widely performed (see, for example, Non-Patent Document 1 below).

Shojiro Kawakami and two others, "Advanced Electronics Series I-16 Optical Fiber and Fiber-Type Device", 1st Edition, Baifukan Co., Ltd., July 10, 1996, p. 217

  However, a spatial coupling system using a lens has a problem in that it is difficult to perform high-efficiency conversion of a mode field because coupling efficiency greatly varies depending on slight differences in the position and size of the lens. On the other hand, a TEC fiber using a taper structure has a problem that it is very difficult to manufacture because the refractive index distribution in the longitudinal direction needs to be controlled by a stretching process during manufacturing.

  In view of the above, an object of the present invention is to provide a mode field conversion fiber that is easy to manufacture and that can easily convert a mode field with high efficiency.

  In order to solve the above-described problem, a mode field conversion fiber according to a first invention includes a core region for propagating light, a cladding region provided so as to surround the core region, and a predetermined core region. A plurality of holes formed in the cladding region so as to surround the gap, and a medium having an arbitrary refractive index filled only at one end side of the holes. And

  A mode field conversion fiber according to a second aspect of the present invention includes a core region for propagating light, a cladding region provided so as to surround the core region, and a boundary between the core region and the cladding region. And a plurality of holes formed in the core region at a predetermined interval, and a medium having an arbitrary refractive index filled only at one end side of the holes. .

In the mode field conversion fiber according to the third aspect of the present invention, in the first or second aspect, the mode field diameter Df on the one end side and the mode field diameter Da on the other end side are expressed by the following formula (1): It is characterized by satisfying the relationship.
D1>Df>Da> D2 (1)
However, D1 is the mode field diameter of the optical fiber or optical device connected to one end side, and D2 is the mode field diameter of the optical fiber or optical device connected to the other end side.

  A mode field conversion fiber according to a fourth invention is characterized in that, in any one of the first to third inventions, the medium has a refractive index equal to or lower than a refractive index of the core region.

  In the mode field conversion fiber according to the fifth invention, in any one of the first to fourth inventions, the refractive index of the medium is smaller on the other end side than on the one end side. It is characterized by.

  A mode field conversion fiber according to a sixth invention is the mode field conversion fiber according to any one of the first to fifth inventions, wherein the temperature coefficient of the refractive index of the medium is higher than the temperature coefficient of the refractive index of the material of the cladding region. It is large.

  The mode field conversion fiber according to the present invention is easy to manufacture and can easily convert the mode field with high efficiency.

  Although an embodiment of a mode field conversion fiber according to the present invention will be described with reference to the drawings, the present invention is not limited to the following embodiment.

[First embodiment]
A first embodiment of a mode field conversion fiber according to the present invention will be described with reference to FIGS. FIG. 1 is a schematic configuration diagram of a mode field conversion fiber, and FIG. 2 is an explanatory diagram of the operation of the mode field conversion fiber of FIG.

  As shown in FIG. 1, the mode field conversion fiber according to the present embodiment includes a core region 11 for propagating light, a cladding region 12 provided so as to surround the core region 11, and a predetermined distance between the core regions 11. A plurality of holes 13 formed in the cladding region 12 so as to surround the medium, and a medium 14 having an arbitrary refractive index n3 filled only on one end side (left side in FIG. 1A) of the holes 13. The inside of the hole 13 on the other end side (the right side in FIG. 1A) is air.

  In such a mode field conversion fiber 10 according to this embodiment, since the medium 14 is filled only on one end side of the hole 13, the optical confinement effect by the hole 13 is lost on one end side. As a result, the mode field diameter Df on one end side filled with the medium 14 becomes larger than the mode field diameter Da on the other end side not filled with the medium 14 (see FIG. 2A).

  By the way, the mode field mismatch loss η generated when the optical fiber or optical device having the mode field diameter D1 and the optical fiber or optical device having the mode field diameter D2 are directly connected is simply expressed by the following formula (11). (See, for example, Non-Patent Document 1 (p. 201) and the like).

η = (2 · D1 · D2) / (D1 ² + D2 ² ) (11)

  On the other hand, an optical fiber or optical device having a mode field diameter D1 is connected to one end side of the mode field conversion fiber 10, and an optical fiber or optical device having a mode field diameter D2 is connected to the other end side of the mode field conversion fiber 10. The mode field mismatch loss ηf generated when connected can be simply expressed by the following equation (12).

ηf = {(2 · D1 · Df) / (D1 ² + Df ² )} ×
{(2 · Da · D2) / (Da ² + D2 ² )} (12)

  Therefore, when the condition satisfies the relationship of the following expression (1), the optical fiber or optical device having the mode field diameter D1 and the optical fiber or optical device having the mode field diameter D2 are passed through the mode field conversion fiber 10. The mode field mismatch loss ηf when connected is made smaller than the mode field mismatch loss η when the optical fiber or optical device having the mode field diameter D1 and the optical fiber or optical device having the mode field diameter D2 are directly connected. (Ηf <η).

D1> Df> Da> D2 (1)

  Therefore, the mode field conversion fiber 10 according to the present embodiment is easy to manufacture and can easily convert the mode field with high efficiency.

  In the mode field conversion fiber 10, if the refractive index n3 of the medium 14 is too larger than the refractive index n1 of the core region 11, the light is localized on the medium 14 side, and the mode field distribution is Gaussian. Therefore, when an optical fiber generally having a characteristic in which the mode field distribution approximates a Gaussian distribution is connected, the mode field mismatch loss becomes large. Therefore, when the mode field conversion fiber 10 is connected to an optical fiber, it is very preferable that the refractive index n3 of the medium 14 is equal to or lower than the refractive index n1 of the core region 11 (n3 ≦ n1).

  Further, for example, as shown in FIG. 2A, in the mode field conversion fiber 10, a region (left side in FIG. 2A) filled with the medium 14 in the hole 13 and the medium 14 in the hole 13. When the boundary between the region not filled with air, that is, the inside of the hole 13 is the air region (the right side in FIG. 2A) is heated by the heating means 100, as shown in FIG. 2B, the inside of the hole 13 is Since the position closer to the heating means 100 is heated so that the temperature becomes higher, the refractive index n3 in the hole 13 filled with the medium 14 becomes smaller as the position is closer to the heating means 100, and the mode field The diameter becomes smaller as the position is closer to the heating means 100. That is, the medium 14 has a refractive index n3 that is smaller toward the other end side than from the one end side.

  Therefore, in the mode field conversion fiber 10, when the boundary between the region on one end side and the region on the other end side is heated, the amount of change in the mode field diameter at the boundary can be extremely reduced. The mode field mismatch loss at the boundary portion can be greatly suppressed, which is very preferable.

  Further, since the mode field conversion fiber 10 is heated not only by the medium 14 but also by the core region 11 and the cladding region 12 due to heating by the heating unit 100, the refractive indexes n1 and n2 change. When the temperature coefficient T3 of the refractive index n3 of the medium 14 is larger than the temperature coefficient T2 of the refractive index n2 of the material of the cladding region 12 (T3> T2), the refractive index n3 of the medium 14 with respect to the refractive index n2 of the material of the cladding region 12 The relative difference can be reduced by the heating, which is very preferable.

Specifically, for example, for a general optical fiber (temperature coefficient: 11 × 10 ⁻⁶ / ° C., (Toshiaki Katagiri, one other person, “Measurement of strain of optical fiber in fusion splicing reinforcement”, electronic IPSJ Journal, Vol.J71-B, No.12, p.1708-1711)), for example, when a general refractive index matching agent is applied to the medium 14 (temperature coefficient: 4 × 10 ^-4 / ° C, (Izumi Mikawa, 3 others, "Fresnel reflection reduction method of optical connector using refractive index matching agent", IEICE Transactions, December 1984, Vol. J67-B, No. 12, p. 1423-1430)), T3 can be about 30 times larger than T2.

  As an example, the relationship between the relative refractive index difference of the medium 14, the connection loss, and the mode field diameter when the core region expansion fiber and the dispersion compensating fiber are connected via the mode field conversion fiber 10 according to the present embodiment. Is shown in FIG. At this time, the diameter 2a (radius a) of the core region 11 is 6.0 μm, the refractive index difference Δ between the core region 11 and the cladding region 12 is 1.0%, and the distance between the center of the core region 11 and the hole 13 is When the distance is c / 2, c / a is 1.2, when the diameter of the hole 13 is d, d / 2a is 1.5, the mode field diameter D1 of the core region expansion fiber is 13 μm, dispersion compensation The mode field diameter D2 of the fiber was 5 μm, and the wavelength of light was 1550 nm. In FIG. 3, the dotted line represents the connection loss when the core region expanding fiber and the dispersion compensating fiber are directly connected.

  As can be seen from FIG. 3, the mode field mismatch loss ηf when the core region expansion fiber and the dispersion compensating fiber having different mode field diameters are connected via the mode field conversion fiber 10 according to the present embodiment is It can be confirmed that the mode field mismatch loss η is smaller (ηf <η) when the core region expanding fiber and the dispersion compensating fiber having different diameters are directly connected.

  If the length of the medium 14 filling the one end side in the air hole 13 is too short, the change of the mode field cannot catch up with the change of the waveguide, which is equivalent to an apparent gap. Therefore, it is preferably about 1000 times or more (1 mm or more) of the wavelength of light.

[Second Embodiment]
A second embodiment of the mode field conversion fiber according to the present invention will be described with reference to FIGS. FIG. 4 is a schematic configuration diagram of the mode field conversion fiber, and FIG. 5 is an operation explanatory diagram of the mode field conversion fiber of FIG. In addition, about the part similar to the case of 1st embodiment mentioned above, by using the code | symbol similar to the code | symbol used by description in 1st embodiment mentioned above, 1st implementation mentioned above is used. The description overlapping with the description in the form is omitted.

  As shown in FIG. 4, the mode field conversion fiber according to this embodiment includes a core region 11 that propagates light, a cladding region 12 that is provided so as to surround the core region 11, and a predetermined distance between the core regions 11. A plurality of holes 13 formed in the cladding region 12 so as to be surrounded by only one end side (left side in FIG. 4B) of only a part of the holes 13 (only the leftmost side and the rightmost side in FIG. 4A) And a medium 14 having an arbitrary refractive index n3 filled in the inside of the other end side (right side in FIG. 4B) of some of the holes 13 and the remaining other holes 13 (FIG. In 4A, the inside of the upper side and the lower side is air.

  That is, in the mode field conversion fiber 10 according to the first embodiment described above, the medium 14 is filled on one end side of all the holes 13, but the mode field conversion fiber according to the present embodiment. In No. 20, the medium 14 is filled on one end side of only some of the holes 13.

  In such a mode field conversion fiber 20 according to this embodiment, as shown in FIG. 5A, the medium 14 is filled only on one end side of the hole 13 located in a predetermined direction (x-axis direction). Therefore, when the diameter 2a (radius a) of the core region 11 is 9 μm, the distance between the center of the core region 11 and the hole 13 is c / 2, c is 13.5 μm, As can be seen from FIG. 5B showing the relationship between the relative refractive index difference and the mode field diameter in a predetermined direction (x-axis direction) and a direction orthogonal to the direction (y-axis direction) when the diameter d is 9 μm. Since the light confinement effect due to the holes 13 is lost only in a predetermined direction on one end side, the mode field distribution on the one end side filled with the medium 14 can be made elliptical, and the medium 14 Stuck It can be the mode field distribution of the other end side which is not filled into a perfect circular shape.

  For this reason, an optical device such as a light source generally having a characteristic that the mode field distribution is elliptical or substantially rectangular is connected to one end side of the mode field conversion fiber 20 so that the mode field distribution becomes a perfect circle. When an optical fiber generally having a characteristic (the mode field diameter does not depend on the direction) is connected to one end side of the mode field conversion fiber 20, an elliptical shape or a substantially rectangular shape output from an optical device such as a light source is formed. Light of mode field distribution can be converted into light of perfect circular mode field distribution, so that light output from optical devices such as light sources can be propagated to optical fibers with very small mode field mismatch loss. it can.

  Therefore, according to the mode field conversion fiber 20 according to the present embodiment, as in the case of the mode field conversion fiber 10 according to the first embodiment described above, it is easy to manufacture and the mode field can be easily converted with high efficiency. can do.

[Other Embodiments]
In the first and second embodiments described above, the case of the mode field conversion fibers 10 and 20 in which a plurality of holes 13 are formed in the cladding region 12 so as to surround the core region 11 at a predetermined interval will be described. However, as another embodiment, for example, as shown in FIG. 6, mode field conversion in which a plurality of holes 33 are formed in the core region 31 at predetermined intervals along the boundary between the core region 31 and the cladding region 32. Even with the fiber 30, it is possible to obtain the same operational effects as those of the first and second embodiments described above.

  In the above-described embodiment, the refractive index distribution is uniform. However, the present invention is not limited to this, and an arbitrary refractive index distribution may be used.

  In the above-described embodiment, the case where the holes 13 and 33 are formed along the circumferential direction so as to be positioned at the apex position of the regular hexagon as shown in FIGS. The present invention is not limited to this, and as another embodiment, for example, a hole is formed along the circumferential direction so as to be positioned at the apex position of another regular polygon, or an annular shape is formed along the circumferential direction. In addition to forming holes periodically along the circumferential direction, such as forming holes so as to form a hole, it is also possible to form holes non-periodically along the circumferential direction. Is possible.

  In the above-described embodiment, as shown in FIGS. 1, 4, 6 and the like, the case where the holes 13 and 33 are formed so as to be in a line along the radial direction has been described. However, the present invention is not limited to this, and it is also possible to form holes so that there are two or more rows along the radial direction.

  Further, in the above-described embodiment, the mode field conversion fiber 10 in which the inside of the other end side (the right side in FIG. 1A) of the holes 13 is air, or the other end side of some of the holes 13 ( In FIG. 4B, the case of the mode field conversion fiber 20 in which the inside of the right side) and the other remaining holes 13 (upper side and lower side in FIG. 4A) are air has been described. The invention is not limited to this, and as another embodiment, for example, the inside of the other end side of the holes, the inside of the other end side of some of the holes, and the inside of the remaining other holes are vacuumed. In the case of the mode-field conversion fiber, the mode-field conversion fiber in which a gas, liquid, solid, or the like having a refractive index smaller than that of the cladding region is put, same It can be obtained Do advantageous effects.

  Since the mode field conversion fiber according to the present invention is easy to manufacture and can easily convert the mode field with high efficiency, it is extremely useful in the optical communication industry such as when optical connection is performed in an optical transmission system or the like. It can be used beneficially.

It is a schematic block diagram of 1st embodiment of the mode field conversion fiber which concerns on this invention. It is action | operation explanatory drawing of the mode field conversion fiber of FIG. 2 is a graph showing a relationship between a relative refractive index difference of a medium, a connection loss, and a mode field diameter when a core region expansion fiber and a dispersion compensation fiber are connected through the mode field conversion fiber of FIG. 1. It is a schematic block diagram of 2nd embodiment of the mode field conversion fiber which concerns on this invention. FIG. 5 is an operation explanatory diagram of the mode field conversion fiber of FIG. 4. It is a schematic block diagram of other embodiment of the mode field conversion fiber which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Mode field conversion fiber 11 Core area | region 12 Cladding area | region 13 Hole 14 Medium 20 Mode field conversion fiber 30 Mode field conversion fiber 31 Core area | region 32 Cladding area | region 33 Hole 100 Heating means

Claims (6)

A core region that propagates light;
A cladding region provided to surround the core region;
A plurality of holes formed in the cladding region so as to surround the core region at a predetermined interval;
A mode field conversion fiber, comprising: a medium having an arbitrary refractive index filled only at one end side of at least a part of the holes. A core region that propagates light;
A cladding region provided to surround the core region;
A plurality of holes formed in the core region at predetermined intervals along a boundary between the core region and the cladding region;
A mode field conversion fiber, comprising: a medium having an arbitrary refractive index filled only at one end side of at least a part of the holes. In claim 1 or claim 2,
The mode field conversion fiber characterized in that the mode field diameter Df on the one end side and the mode field diameter Da on the other end side satisfy the relationship of the following formula (1).
D1>Df>Da> D2 (1)
However,
D1 is the mode field diameter of the optical fiber or optical device connected to one end side,
D2 is the mode field diameter of the optical fiber or optical device connected to the other end side. In any one of Claims 1-3,
The mode field conversion fiber, wherein the medium has a refractive index equal to or lower than a refractive index of the core region. In any one of Claims 1-4,
The mode field conversion fiber, wherein the refractive index of the medium is smaller toward the other end side than from the one end side. In any one of Claims 1-5,
The mode field conversion fiber, wherein the temperature coefficient of the refractive index of the medium is larger than the temperature coefficient of the refractive index of the material of the cladding region.

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