JPH04152305A - Fiber for producing mode converter and production thereof and mode converter using this fiber - Google Patents

Abstract

PURPOSE:To obtain the mode converter which decreases the connection losses of an optical waveguide and an optical fiber by providing a 2nd core having a low refractive index and a circular section around a 1st core having a square section and providing a low-refractive index clad around this core. CONSTITUTION:The 1st core 12 in the large-diameter part of the mode converter is disposed to face the optical waveguide 4. The converter and the optical waveguide type parts are then connected. The 2nd core 14 in the small-diameter part of the converter is disposed to face the core 8 of the optical fiber and is connected thereto. Light propagates in the core 12 when the light is emitted from the waveguide side. The light propagates in the core 14 upon arrival at the small-diameter part. The propagated light of the core 14 is coupled to the core 8 of the fiber. Mode conversion is reversibly executed when the light propagates from the fiber side to the waveguide side. The connection losses of the optical waveguide and the optical fiber are decreased in this way.

Description

[Detailed Description of the Invention] Overview Regarding a fiber for manufacturing a mode converter, a method for manufacturing the same, and a mode converter using the above-mentioned fiber, the main purpose is to reduce connection loss between an optical waveguide and an optical fiber, and the present invention relates to a mode converter manufacturing method. The fiber for use includes a first core having a rectangular cross section;
It is configured to include a second core having a circular cross section with a lower refractive index than the core, and a cladding provided around the second core and having a lower refractive index than the second core.

INDUSTRIAL APPLICATION FIELD The present invention relates to a fiber for manufacturing a mode converter, a method for manufacturing the same, and a mode converter using the above-mentioned fiber.

In optical communications and other fields, waveguide-type optical components formed by forming an optical waveguide on a substrate have come to be used as system components. Waveguide-type optical components have the advantage of being more easily mass-produced and smaller than bulk-type optical components, but on the other hand, when connected directly to an optical fiber used as an optical transmission line, coupling There is a problem that the efficiency is not necessarily good, and it is desired to deal with this problem.

Prior art FIGS. 6(a) and 6(b) are diagrams showing cross-sectional structures of an optical waveguide and an optical fiber, respectively. In FIG. 6(a),
2 is a substrate, 4 is a core layer, and 6 is a cladding layer, and the refractive index of the core layer 4 is higher than that of the cladding layer 6. Due to this difference in refractive index, light propagates within the core layer 4. In FIG. 6b), 8 is a core and 10 is a cladding, and the refractive index of the core 8 is set higher than that of the cladding 10. Even in an optical fiber with this structure, light is guided through the core 8 by the same waveguide principle as in an optical waveguide.
propagate within.

Problems to be Solved by the Invention In a guiding waveguide having the structure shown in FIG. 6(a), the mode distribution in the core cross section parallel to the substrate and the mode distribution in the core cross section perpendicular to the substrate are generally different. . On the other hand, in an optical fiber having the structure shown in FIGS. 6 and 7, the mode distribution in the core cross section including the core center is uniform.

Therefore, when an optical waveguide with such a configuration is directly connected to an optical fiber, a loss occurs, and according to experiments, the loss is 1 to 3.
It is about dB.

An object of the present invention is to provide a mode converter that can reduce connection loss between a leading waveguide and an optical fiber.

Another object of the present invention is to provide a fiber for manufacturing a mode converter, which is a base material for manufacturing this mode converter.

Still another object of the present invention is to provide a method for manufacturing a fiber for manufacturing a mode converter.

Means for Solving the Problems The fiber for manufacturing a mode converter of the present invention includes a rod 1 core having a rectangular cross section, a rod provided around the first core, and a rod 1 having a rectangular cross section.
It is configured to include a second core having a circular cross section with a refractive index lower than that of the core, and a cladding provided around the second core and having a refractive index lower than that of the second core.

The method for manufacturing a fiber for manufacturing a mode converter of the present invention includes a first step of forming a glass layer as a second core in a quartz reaction tube by CVD method, and a second step in the quartz reaction tube on which the glass layer is formed. The method includes a second step in which a prismatic glass rod serving as one core is inserted and collapsed to form a preform, and a third step in which the preform is melt-spun.

In the mode converter of the present invention, the above-described fiber for manufacturing a mode converter is heated and stretched so that the cross-sectional areas of the first and second cores decrease continuously in the longitudinal direction.

Function: The mode converter of the present invention includes a first core having a rectangular cross section, and a second core having a circular cross section and having a lower refractive index than the first core, which is provided around the first core. The fiber for manufacturing mode converters is heated and stretched so that the cross-sectional area of the first and second cores decreases continuously in the longitudinal direction, so that the first core on the larger diameter side faces the optical waveguide. By arranging this mode converter so that the second core on the small diameter side faces the core of the optical fiber, it is possible to connect the optical waveguide and the optical fiber with low loss.

Example Embodiments of the present invention will be described below based on the drawings.

FIG. 1 is a diagram showing a cross-sectional structure of a fiber for manufacturing a mode converter in an embodiment of the present invention. with a square cross section′
! In this example, the size of the J1 core 12 was 7 μm both vertically and horizontally. The second core 14 having a circular cross section surrounding the first core 12 has a diameter of 30 μm. The cladding 16 surrounding the second core 14 has a circular cross section in this embodiment, and its diameter is 370 μm.

FIG. 2 is a diagram showing the refractive index distribution in the radial direction of the fiber for manufacturing the mode converter shown in FIG. The vertical axis represents the refractive index, and the horizontal axis represents the position in the radial direction. The first core 12, the second core 14, and the cladding 16 each have a uniform refractive index.
The refractive index of the core 12 is n, and the refractive index of the second core 14 is n2.
The refractive index of the cladding 16 is n3 (n, < 12 < n+
). The relative refractive index difference between n and n2 is 0.1%, and n
The relative refractive index difference between 2 and η3 was set to 0.2%.

A specific example of a method for manufacturing the fiber for manufacturing a mode converter shown in FIG. 1 will be explained with reference to FIG.

First, as shown in FIG. 3(a), a soot-like core glass 14A, which is to become a second core and has a refractive index higher than that of the quartz reaction tube, is formed in a quartz reaction tube 16A by the CVD method. . Next, as shown in FIG. 3(b), the soot-like core glass is vitrified by heating to form a glass layer 14.
B, and after cooling, the prismatic glass rod 12Δ serving as the first core is inserted into the quartz reaction tube. The refractive index of the prismatic glass rod 12A is set higher than the refractive index of the glass layer 14B. Thereafter, collapse is performed with the prismatic glass rod 12A still inserted into the quartz reaction tube to obtain a preform having a cross-sectional shape similar to that of the fiber for producing a mode converter, as shown in FIG. 3(C). In this preform, 12A is a part corresponding to the first core, 14C is a part corresponding to the second core,
16B is a portion corresponding to the cladding. Then, by melting and spinning this preform, a fiber for manufacturing a mode converter can be obtained.

FIG. 4 is an explanatory diagram of a method for manufacturing a mode converter using the fiber for manufacturing a mode converter shown in FIG. In this example, using a burner 18 such as an H202 burner, the fiber 20 for producing a mode converter shown in FIG. 1 is drawn while being partially heated. After adjusting the diameter to about ⅓ of the outer diameter, it is cooled and cut approximately at the center of the extended portion indicated by the broken line A. According to this method, the fiber has a shape in which the cross-sectional area decreases continuously in the longitudinal direction, so it is possible to provide a mode converter with small loss in the tapered portion (stretched portion). In this example, the first core at the minimum diameter part to be coupled to the optical fiber is 2 μm x 2 μm.
The second core diameter is 10 μm, and the outer diameter is 125 μm.
It was μm.

How this mode converter is used will be explained with reference to FIG.

In this embodiment, the mode converter manufactured by the manufacturing method shown in FIG. 4 is used to connect the optical waveguide and optical fiber shown in FIG. 6. That is, the mode converter and the optical waveguide type component are connected using an adhesive or the like so that the first core in the large diameter part of the mode converter faces the optical waveguide 4, and the second core in the small diameter part of the mode converter is connected using an adhesive or the like. The mode converter and the optical fiber are connected, for example, by bridging so that the core faces the core of the optical fiber. Assuming that light is guided from the optical waveguide side to the optical fiber, the light emitted from the optical waveguide first propagates through the first core in the mode converter.
When this light reaches the small diameter part of the mode converter, the first core in the small diameter part is approximately 2 μm and has lost its light confinement ability, so the light propagates in the small diameter part using the second core as the core. Become. The propagating light of the second core is then coupled to the core of the optical fiber with high efficiency. Furthermore, when light is propagated from the optical fiber side to the optical waveguide side, mode conversion is performed reversibly, allowing highly efficient optical coupling. The coupling loss in this example was 0.3 dB.

In addition to the manufacturing method of the fiber for manufacturing mode converters shown in Fig. 3, after forming the preform, a square hole is made in the center, and a prismatic glass rod is inserted into this hole to form the rod. A method of melting and spinning using an in-tube method may also be adopted.

As described in detail, according to the present invention, it is possible to reduce connection loss between an optical waveguide and an optical fiber.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the cross-sectional structure of a fiber for manufacturing a mode converter in an embodiment of the present invention, and FIG. 2 is a diagram showing the refractive index distribution in the radial direction of the fiber for manufacturing a mode converter shown in FIG. 1. , FIG. 3 is an explanatory diagram of a method for manufacturing the fiber for manufacturing a mode converter shown in FIG. 1, and FIG. 4 is an illustration of manufacturing a mode converter using the fiber for manufacturing a mode converter shown in FIG. 1. FIG. 5 is an explanatory diagram of how the mode converter manufactured by the manufacturing method of FIG. 4 is used. FIG. 6 is an explanatory diagram of the cross-sectional structure of an optical waveguide and an optical fiber. I2...first core, 14...second core, 16...cladding.

Claims (1)

[Claims] 1. A first core (12) having a rectangular cross section, and a circular cross section provided around the first core (12) and having a lower refractive index than the first core (12). a second core (14); and a second core (14) provided around the second core (14).
4) A fiber for manufacturing a mode converter, comprising a cladding (16) having a lower refractive index than 4). 2. A method for manufacturing a fiber for manufacturing a mode converter according to claim 1, comprising: placing a glass layer (1) serving as a second core in a quartz reaction tube (16A);
4B) by the CVD method, and the quartz reaction tube (14B) on which the glass layer (14B) is formed.
6A), a second step of inserting a prismatic glass rod (12A) serving as a first core and collapsing it into a preform, and a third step of melt-spinning the preform. A method for manufacturing a fiber for manufacturing a mode converter, characterized by: 3. The fiber for producing a mode converter according to claim 1 is heated and stretched so that the cross-sectional area of the first and second cores (12, 14) decreases continuously in the longitudinal direction. Features a mode converter.