JPH0752248B2 - Optical waveguide mode scrambler - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical waveguide mode scrambler for uniformly splitting and combining an incident light beam in a multimode waveguide type optical waveguide, and more particularly to an optical branching / scrambler. Even if a large number of connecting portions are connected in a vertical direction, the branching / coupling can be uniformly performed at a desired ratio at each branch / coupling point.

(Prior Art) An optical communication system that has been put into practical use is a pair of optical fibers that connect a light source that generates signal light to a light receiver that receives the signal light and converts the signal light into an electrical signal with one optical fiber. 1 is the simplest form. By further developing such one-to-one optical communication, bidirectional optical communication in which information is bidirectionally transmitted between a light source and a light receiver by a single optical fiber, one-to-many broadcast type optical communication, and more than one In order to develop a multi-to-many local area network (LAN), that is, an optical communication network such as an intra-regional communication network, the light from the optical fiber forming the optical transmission line is branched to another optical fiber. In addition, an optical branching / coupling circuit that joins / combines light from other optical fibers, and branches the light into a large number of optical fibers at once is indispensable for the configuration of the optical communication network.

The optical fiber transmission line introduced at the first stage in the above-mentioned network optical communication is a multimode optical fiber in which the connection between the optical fibers is easy, and in particular, a distributed-refraction-type optical fiber capable of performing optical transmission in a relatively wide band. It is considered to be a mode optical fiber. This distributed index type multimode optical fiber is of a parabolic type in which the distribution of the refractive index in the core of the optical fiber decreases in proportion to approximately the square of the distance r from the central axis. The following three types have been conventionally developed as optical branching / coupling circuits for such multimode optical fibers.

(1) As shown in FIG. 1 (a), an optical fiber type optical branching / coupling device in which a plurality of optical fibers are twisted and stretched while being heated to form a fusion taper portion 1.

(2) As shown in FIG. 1 (b), the incident light from the optical fiber is converted into a parallel light beam by using a minute lens 2 such as a branch refractive index rod lens, and the light is branched into two through a semi-transparent mirror 3. An optical splitter / combiner that combines individual components to collect light again on the optical fiber by using the minute lenses 4 and 5.

(3) An optical waveguide type optical branching / coupling device using an optical waveguide 7 patterned on a flat substrate 6 as shown in FIG. 1 (c).

Of these, the optical fiber type optical branching / combining device (1) is a kind of glass work and is not suitable for mass production, and the component combination type optical branching / combining device (2) is used for a large number of optical components. Since it requires a lot of labor and time for assembly and adjustment, it is not suitable for mass production as well, but the optical waveguide type optical branching / coupling device uses the planar technology often used in the semiconductor industry. It has excellent reproducibility of properties and characteristics, and seems to be suitable for mass production. However, even when an optical branching / combining device is manufactured using this optical waveguide, the branching ratio changes depending on the mode excitation distribution in the optical waveguide at the time of incidence at the branching part, so the multimode optical fiber and the optical waveguide When there is axial misalignment at the connection part with, or when there is an optical coupling part immediately before the optical branching part and the mode distribution is disturbed, and further when such optical branching part or optical coupling part is connected in cascade. In addition, there is a problem that a stable branching ratio cannot be obtained because the mode distribution is different in each case.

In order to solve such a problem, the present inventors previously stated that
As shown in FIG. 2, a zigzag mode scrambler in which a zigzag type optical waveguide 7 is formed on a flat substrate 6 and mode bending is performed by bending the zigzag type optical waveguide 7 is inserted in front of an optical branching portion. An optical waveguide device in which the distribution is close to a steady mode distribution and a stable branching ratio is obtained has been proposed by Japanese Patent Laid-Open No. 60-143304.

Next, the inventors of the present invention aim to analyze and optimally design the branching / coupling characteristics of the optical branching / combining device with a mode scrambler as described above, and analyze the branching ratio and loss by ray tracing and display the phase space. Was developed so that the analysis of the branching / coupling characteristics in the optical branching / coupling device as described above can be carried out reliably and easily. Therefore, the former ray tracing is, as shown in FIG. 3, replacing the mode of the propagation mode of light at the incident end of the optical waveguide with a ray, calculating the trajectory of the ray, and outputting each after branching / coupling. The branching ratio is obtained by counting the number of rays at the edge, and the number of rays at the time of incidence
The loss is obtained by taking the ratio of I _T and the total number of rays I _R and I _L at the time of emission. In addition, the latter phase space display means, as shown in FIG.
As shown in FIG. 4 (b), the trajectory of the above-mentioned ray in the real space represented by the coordinates is represented by the distance x from the center of the optical waveguide at the exit end of the optical waveguide and the inclination (= dx / dz) at that position. Rays are displayed as points according to the coordinates taken on the horizontal axis and the vertical axis, respectively, and if the points representing the outgoing rays are evenly distributed on the phase space coordinates, the mode distribution becomes a stationary mode distribution. It is designed so that it can be considered close.

(Problems to be Solved by the Invention) By using the above-described analysis method developed by the present inventors,
The optical branching / combining device with the zigzag mode scrambler is configured by the narrow band step index optical waveguide in which the refractive index in the core region differs from the refractive index in the glad region in a uniform distribution. As a result of analyzing the mode distribution in the case, the mode distribution scattered in the real space coordinates as shown in FIG. 5 (a) is shown in FIG. 5 (b). It was found that the points corresponding to the modes in the phase space were scattered almost uniformly and the mode distribution was made uniform.

On the other hand, the refractive index distribution in the core region is 2
The result of analyzing the mode distribution when an optical branching / combining device with a zigzag mode scrambler is similarly constructed with a relatively wide band distributed index optical waveguide that decreases in proportion to the power is the real space coordinates. It is shown in FIG. 6 (a) when expressed by, and is shown in FIG. 6 (b) when expressed in phase space coordinates.
As can be seen from comparison with FIGS. 5 (a) and 5 (b), the mode distribution has a significant bias and is not made uniform at all. Therefore, if the light having such a non-uniform mode distribution is further branched by the optical branching portion,
The splitting ratio at the optical splitting part did not become 1: 1 and the splitting state became unstable.

As is clear from the above description, the mode scrambler based on the zigzag type optical waveguide that was previously developed by the present inventors is very effective for a narrow band step index type optical waveguide, There is a problem that it has no effect on the distributed index optical waveguide.

An object of the present invention is to solve the above-mentioned conventional problems and to provide a new optical waveguide mode scrambler that exhibits a sufficient effect of mode mixing even for a broadband distributed index optical waveguide. .

(Means for Solving Problems) However, the optical waveguide mode scrambler of the present invention skillfully utilizes the fact that mode conversion always occurs at the merging portion of the optical waveguide, so In order to ensure that the mode-mixing effect appears even in an optical waveguide in which mode conversion does not remarkably occur only by bending it in a zigzag shape, the optical waveguide is configured so that merging and branching are alternately repeated. In contrast to the conventional zigzag type, it should be called a bellows type.

The multimode optical waveguide of the present invention is an optical coupling to a multimode optical waveguide composed of at least two optical waveguides so that the coupling and branching of incident light are alternately repeated between the entrance end and the exit end. It is characterized in that it is composed of a three-dimensional optical waveguide in which the sections and the optical branching sections are configured alternately, and is configured so that the multimode incident light beams can be uniformly combined and repeatedly repeated.

(Operation) Therefore, according to the present invention, the optical branching / coupling of an optical circuit having an optical branching circuit, an optical merging circuit, an optical star coupler, etc. In this case, the mode distribution can be made sufficiently uniform even when a distributed index type optical waveguide is used.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings.

First, a schematic structure of the optical waveguide mode scrambler of the present invention is schematically shown in FIG. 7 (a), and a mode of refractive index change in the branching / coupling portion thereof is schematically shown in FIG. 7 (b). As shown by the alternate long and short dash line in the drawing, the central axis of the optical waveguide forming the mode scrambler intersects and repeats branching and coupling, and the outer shape of the optical waveguide is a bellows type, that is, a bellows type. The mode distribution when the bellows type mode scrambler is applied to an optical branching / combining device composed of a distributed index type optical waveguide is shown in FIG.
It is shown in FIG. The illustrated bellows-type mode scrambler has only one branching and coupling, but it can be seen that even a single branching and coupling causes a considerable mode mixing. When the number M of branching / coupling of the central axis of the optical waveguide in the bellows type mode scrambler is increased, as shown sequentially in FIGS. 9 (a) to 9 (d), the mode distribution of the phase space coordinates is shown. The homogenization can be progressively increased.

Such an optical waveguide mode scrambler of the present invention can be applied to various multimode waveguide type optical branching / coupling circuits, and particularly in an optical waveguide in which the refractive index distribution in the core is close to the square distribution. In the conventional zigzag type mode scrubber as shown in Fig. 4, the action effect of mode mixing cannot be obtained, whereas in the bellows type mode scrambler of the present invention which alternately repeats branching and coupling, the action effect of remarkable mode mixing is obtained. Is obtained. For example, as shown in FIG. 10, when the bellows type mode scrambler according to the present invention is applied to an optical branching / coupling circuit by a diffusion type multimode optical waveguide 7 in which metal ions are injected into a glass substrate 6 as an electric field, The advantage is that it can be easily integrated with other optical branching circuits or optical coupling circuits of the type.

Further, as described above, the branching / coupling portion is not limited to the optical waveguide type optical circuit using the flat substrate, and as shown in FIG. 11, the optical fiber is fused or the side surface is polished to be coupled. Also in the optical fiber type optical branching / coupling circuit formed with, it is easy to repeat branching and coupling a plurality of times, and a bellows type mode scrambler can be substantially configured.

In the optical branching / coupling circuit using the mode scrambler according to the present invention shown in FIGS. 10 and 11, the light beam incident from the input end 1 or 2 is branched to the output end 1 or 2, or , The light beams incident from the input ends 1 and 2 can be combined with the output end 1 or 2.

(Effects of the Invention) As is clear from the above description, in the optical branching / coupling circuit including the multimode optical waveguide, the branching ratio is required unless the mode scrambler that performs the mode mixing action is placed in front of the branching / coupling portion. Changes depending on the mode distribution, and when branching / coupling is repeated, the mode distribution is disturbed each time and even branching / coupling cannot be performed, and even if a conventional zigzag mode scrambler is placed in front, Although the nonuniformity of the distribution was not necessarily improved, if the bellows-type mode scrambler according to the present invention is placed in front, the mode distribution immediately before each branching / coupling portion is brought close to the state of the steady mode distribution and the uniform optical branching is performed. -The remarkable result is that the combination can be performed.

[Brief description of drawings]

1 (a), (b), and (c) show various conventional optical branches,
Fig. 2 is a diagram showing the configuration of each coupler, Fig. 2 is a perspective view showing the configuration of a conventionally proposed zigzag mode scrambler, and Fig. 3 is a diagram showing the principle of optical branching / coupling characteristic analysis by ray tracing. 4 (a) and 4 (b) are characteristic curve diagrams showing modes of mode distribution expression in real space coordinates and phase space coordinates, respectively, and FIGS. 5 (a) and 5 (b) are optical branching by a staircase refractive index type optical waveguide. Diagrams showing the mode distribution mode of the conventional zigzag mode scrambler of the coupler in terms of real space coordinates and phase space coordinates, respectively. FIGS. 6 (a) and 6 (b) are optical branches by distributed index optical waveguides. -A diagram showing the mode distribution mode of the conventional zigzag mode scrambler of the combiner in real space coordinates and phase space coordinates, respectively, and Figs. 7 (a) and 7 (b) are according to the present invention. Schematic configuration of the bellows-type mode scrambler and a part of the light branching and
FIGS. 8 (a) and 8 (b) are schematic diagrams each showing the mode of the refractive index change in the coupling portion, which show a bellows mode scrambler according to the present invention of an optical branching / coupling device using a distributed index optical waveguide. Diagrams showing examples of modes of mode distribution in real space coordinates and phase space coordinates, respectively, and FIGS. 9 (a) to 9 (d) also show an increase in the number of optical branching / coupling portions in the bellows type mode scrambler. FIG. 10 is a diagram sequentially showing modes of mode distribution in terms of phase space coordinates, and FIG. 10 schematically shows an example of a schematic configuration of an optical waveguide type optical branching / coupling device using a bellows type mode scrambler according to the present invention. FIG. 11 is a perspective view schematically showing an example of a schematic configuration of an optical fiber type optical branching / coupling device using a bellows type mode scrambler according to the present invention. 1 …… Melting taper part 2,4,5 …… Distributed index rod lens 3 …… Half mirror 6 …… Glass flat plate substrate 7 …… Diffusion type multimode optical waveguide

Claims (3)

[Claims] 1. A multi-mode optical waveguide comprising at least two optical waveguides, and an optical coupling section and an optical coupling section for alternately repeating coupling and branching of incident light between the entrance end and the exit end thereof. An optical waveguide mode scrambler comprising a three-dimensional optical waveguide in which branching portions are alternately configured, and configured to repeat multi-mode incident light beams uniformly and repeatedly. 2. The optical waveguide mode scrambler according to claim 1, wherein the optical branching / coupling portion is formed by introducing electric fields of metal ions into a glass substrate. 3. The optical waveguide mode scrambler according to claim 1, wherein the optical branching / coupling portion is formed by melting or polishing a plurality of optical fibers and coupling them together.