JPH0215843B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical branching/coupling device that reduces optical loss due to branching or coupling in an optical transmission line.

In conventional optical transmission systems that include an optical branch/coupler, the main transmission line and the optical branch/coupler are constructed of optical fibers with the same NA (numerical aperture). In such a configuration, insertion loss of the optical branch/coupler occurs as described below. The block diagram of the optical branching coupler in which the flat parts of the drawing are joined together is shown. The flat part is obtained by polishing the side surface. In the figure, 1 and 1' are the optical fibers that constitute the main transmission line, and 2 is the maximum polishing position A and the center of curvature O.
This is the plane that includes the line connecting . Now, if we define the propagation angle of light as the angle it makes with the fiber axis, the propagation angle of light that enters other optical fibers on the left side of plane 2 will be large, and on the contrary, the propagation angle will be large for light that enters other optical fibers on the right side of plane 2. The propagation angle of the incident light becomes small. For example, consider light ray 3 propagating through optical fiber 1 at a propagation angle θ ₁ , and if this light enters optical fiber 1' from position B, there is a difference between the propagation angle θ ₂ at optical fiber 1' and There is a relationship of θ ₂ = θ ₁ + 2α ₁ (however, α ₁ = ∠
AOB). Similarly, if light with a propagation angle θ ₃ enters the fiber 1' from position C, the propagation angle θ ₄ is θ ₄ =θ ₃ −2α ₂
becomes. (However, α ₂ = ∠AOC).

As explained above, the propagation angle of the light propagated into another optical fiber on the left side of the plane 2 increases, and the light propagated near the maximum propagation angle θ _nax of the optical fiber is converted into a radiation mode.

Therefore, the higher-order mode light that has propagated through the main transmission line cannot propagate within the optical branch/coupler, resulting in loss. This situation is shown in FIG. However, if the NA of the fiber is even larger, the light that has been in the radiation mode will be converted to a lower-order mode by mode conversion, as shown by the broken line 100 in Figure 1.
will be transmitted. The figure shows a core diameter of 100 μm.
A 3-dB optical branch composed of fibers with an NA of 0.28.
This is the dependence of the coupler loss on the light incident angle. The light source is
When using a He-Ne laser and changing the incident angle θ at the terminal 4 as shown in Fig. 1, the terminal 5,
The output from 7 was measured and the loss was calculated using the following formula.

Loss (%) = P ₅ + P ₇ / P ₄ × 100 P ₄ : Input to terminal 4 P ₅ , P ₇ : Output from terminals 5 and 7 Note that the incident angle in Figure 1 is the output from the fiber into the air. It is shown at the angle when Since the incident angle θ is approximately proportional to the propagation angle, as shown in Figure 2, in the type of optical branching coupler made by polishing and joining two fibers, higher-order mode light is converted to radiation mode. It can be seen that this is the cause of losses. The dotted line 8 shows the critical angle calculated from NA.

The present invention provides an optical transmission device that can propagate even high-order mode light as described above without loss, and the present invention will be described below based on embodiments along with drawings.

FIG. 3 shows an example of an implementation configuration of the present invention. 9,
Reference numeral 9' indicates an optical fiber constituting the main transmission line, 10 indicates an optical branching/coupling device having an NA larger than that of the optical fibers 9 and 9', X indicates a transmitting station, and Y and Z indicate a receiving station. Suppose that receiving station Y is now located between X and Z. Generally, as the NA of a fiber increases, the transmission band decreases. Therefore, the maximum value of NA of the main transmission path is limited by the transmission band. Since the optical branch/coupler 10 has a very short transmission path length,
Even if the NA of the fiber is increased in this portion, the overall transmission band will not be reduced. For example, if optical fibers 9 and 9' with a core refractive index of 1.54 and an NA of 0.2 are used as the main transmission line, and the optical branch/coupler 10 is configured with an optical fiber with an NA of 0.28, the transmitted light will be inside the fiber. The maximum propagation angle at is about 7.5°,
The angle of incidence into the air is approximately 11.5°. Therefore, as can be seen from FIG.
There is almost no loss when entering.

Next, it will be shown that there is little loss even when the signal enters the transmission line 9' again from the branch. A curve 12 in FIG. 4 indicates the output end faces 5 and 7 when light is input from the LED light source from one terminal face 4 of the optical splitter/combiner configured as shown in FIG.
This shows the dependence of the total output on the exit angle.
In the figure, the vertical axis is normalized by the value of the exit angle of zero. Similarly, curve 11 is the output angle dependence when LED light is incident on a single fiber of the same type as the optical branching coupler used for measurement of curve 12. Curve 12 in the figure
As can be seen from the comparison between curve 11 and curve 11, the light emitted from the optical splitter coupler 10 (curve 12) has a smaller proportion of higher-order mode light than the output light from a single fiber (curve 11). There is. Curve 13 is a normalized value obtained by dividing the output from optical splitter coupler 10 by the output of a single fiber of the same type as the fiber used for the splitter. From this figure, for example, the exit angle
It can be seen that at 15 degrees, the output of the optical splitter is reduced to about 30% compared to a single fiber. That is, it is shown that the low-order mode light incident on the optical branching coupler once becomes a high-order mode, but is converted back to a low-order mode.

As explained above, even if the output from the optical splitter/coupler 10 enters the main transmission line 9', the loss at that position is small. In an experiment, when the main transmission lines 9, 9' and the optical branch/coupler were all made of optical fiber with an NA of 0.28, the loss due to the optical branch/coupler was 0.9 dB, but when the optical branch/coupler was made with an NA of 0.34. When the main transmission line was made with NA of 0.28, the loss of the optical branch/coupler was reduced to 0.5dB.

In particular, in a single-line bidirectional communication system as shown in FIG.
When constructed with optical fibers with a larger NA than
Branch losses are further reduced. Optical splitter/combiner 16
The main transmission line 17 is an optical fiber with an NA of 0.34.
When each is composed of optical fibers with NA of 0.28,
Branch loss was 0.24dB. On the other hand, regarding coupling, an optical branching coupler using a fiber with an NA of 0.34 has a 0.68 dB stronger intensity incident on the main transmission line 17 than an optical branching coupler using an optical fiber with an NA of 0.28. Summer. That is, the optical splitter/combiner 16
It is better to configure the optical branching coupler with a fiber with an NA larger than the NA of the main transmission line fiber than to configure it with an optical fiber with the same NA as the optical fiber of the main transmission line. The transmission loss from to the light receiving sections 15' and 15 is reduced by 1.32 dB.

As explained above, the optical branching/coupling device of the present invention has a NA larger than that of the optical fiber of the main transmission path.
Since it is composed of NA optical fiber, transmission loss can be reduced without narrowing the transmission band. In particular, transmission loss is significantly reduced when long-distance transmission after branching is not required, such as in single-wire bidirectional communication.

[Brief explanation of drawings]

Fig. 1 is a principle diagram of the optical splitter/coupler, Fig. 2 is a characteristic diagram showing the dependence of loss on incident angle, Fig. 3 is a block diagram showing an embodiment of the present invention, and Fig. 4 is a diagram of the output light. FIG. 5 is a characteristic diagram showing the output angle dependence, and is a configuration diagram showing another embodiment of the present invention. 1,1'...optical fiber for optical branching coupler, 9,
9', 17... Optical fiber for main transmission line.

Claims (1)

[Claims] 1. In an optical transmission line device comprising an optical branch/coupler and a main transmission line, which are formed by fixing two curved optical fibers each having a flat part formed on a convex part with the flat parts facing each other, the above-mentioned An optical transmission device characterized in that the numerical aperture of the optical fiber constituting the optical branch/coupler is larger than the numerical aperture of the optical fiber constituting the main transmission path connected to the optical branch/coupler. .