JPS5981B2 - Mode conversion optical circuit - Google Patents

Description

Detailed Description of the Invention This invention converts high-order mode light of each mode propagated through a multimode optical fiber into low-order mode light, and converts the low-order mode light into high-order mode light, and The present invention relates to a mode conversion optical circuit having a mode conversion function that allows propagation through an optical fiber.

A multimode optical fiber has a band limit due to the fact that each propagation mode has a different group velocity. For example, an optical fiber 1 with a stepped refractive index distribution consisting of a cladding (refractive index n2) 2 and a core (refractive index nl) 3 as shown in FIG.
Then, the angle between the optical fiber center axis and the light ray is small, as represented by light ray 4, and the one where the angle between the optical fiber center axis and the light ray is large, as represented by light ray 5.
Ray 5 takes longer to propagate through the same length of fiber. This results in a band limit due to the difference in group velocity of each mode of the multimode optical fiber. As a method of alleviating this band limitation, as shown in FIG. 2, a light ray (for example, light 5
) into a propagating ray that makes a small angle with the central axis of the optical fiber 9, and a mode that converts a ray that has propagated making a small angle with the central axis (example ray 4) into a propagating ray that makes a large angle with the central axis. If the conversion optical circuit 6 is inserted, the propagation time of each light beam propagating along the entire length of the optical fiber transmission line is almost the same, and as a result, the band limit of the optical fiber transmission line is significantly relaxed compared to the case where the mode conversion optical circuit 6 is not inserted. can. A conventional mode conversion optical circuit having such a function is shown in FIG.

In FIG. 3, reference numerals 10 and 11 are gradient index lenses having ten period lengths, and their focal planes coincide with the end surfaces of the lenses.
Lenses 10 and 11 have respective optical axes 01-02 and 03-04.
are placed off-axis as shown in FIG. 8, 9
is an optical fiber whose end face is placed on the focal plane of lenses 10 and 11.

Further, the two optical fiber end faces are placed at mutual imaging positions formed by lenses 10 and 11. Among the light rays that have propagated through the optical fiber 8, those that make a large angle with the central axis are emitted into the lens 10 at a large exit angle, as shown by light rays 12 and 14. Among these, the light ray 12 is incident on the central axis 03-04 of the lens 11 because the lens 11 is placed with an axis shifted from the lens 10, and therefore becomes a light beam parallel to the optical axis by the lens 11 and is connected to the optical fiber 9. incident on the On the other hand, the light that has propagated within the optical fiber 8 almost parallel to the central axis of the optical fiber enters the lens 10 parallel to the optical axis as represented by the ray 13, and the optical axis of the lens 11 is 01-0.
The light exits the lens 10 from above 2 and enters the lens 11. Since the lenses 11 and 10 are placed with their axes shifted, this light ray will be incident at a location away from the optical axis 03-04 of the lens 11, and therefore the lens 11 will make a large angle with the optical axis. It is converted into a light beam and is emitted into the optical fiber 9. In this way, among the light that has exited the optical fiber 8 through this mode conversion optical circuit, the light rays that have propagated at a large angle with the central axis of the optical fiber 8, such as the light ray 12, are located at the center of the optical fiber 9. Light rays that make a small angle with the optical fiber axis, and light rays that have propagated at a small angle with the central axis of the optical fiber 8, such as ray 13, are converted into light rays that make a large angle with the central axis of the optical fiber 9. . However, in this mode conversion optical circuit, among the light that has propagated within the optical fiber 8 while making a large angle with the central axis, as represented by the ray 14, the inside of the lens 10 is moved away from the optical axis 02-04 of the lens 11. Since the optical axes of lens 10 and lens 11 are misaligned, the light emitted from lens 1
At one output end, the light is converted into a light beam that makes an even larger angle with the optical axis, which is undesirable. Further, for example, if the light ray 14 is a light ray propagated at the critical angle of the optical fiber, this light cannot become a propagation ray within the optical fiber 9, causing a loss. In order to eliminate these drawbacks, the present invention includes a gradient index lens having a reflective surface on one of the meridional planes, which is disposed off-axis, and will be described in detail below with reference to the drawings.

FIG. 4 shows an embodiment of the mode conversion optical circuit according to the present invention, and FIG. 5 is a sectional view thereof.

In the figure, reference numerals 15 and 16 denote gradient index lenses having reflective surfaces, each of which has a length of ten cycles, so that the focal plane of each lens is on the end surface of the lens. 15s, 16s
is a meridian plane forming a reflecting surface, and 8 and 9 are optical fibers.

The lenses 15 and 16 are arranged so that their reflective surfaces 15s and 16s are substantially parallel and their optical axes 01-02 and 03-04 are offset by a required amount. Now, the operation of this mode conversion circuit will be explained with reference to FIG. Of the light rays that propagate through the optical fiber 8, those that make a large angle with the optical fiber center axis are emitted into the lens 15, as shown by light rays 12 and 14, making a large angle with the optical axis. Of these, ray 1
Since the lens 16 is placed off-axis from the lens 15, the light beam 2 is incident on the central axis 03-04 of the lens 16, and is turned into a light beam parallel to the optical axis by the lens 16 and is incident on the optical fiber 9. This becomes a light ray that propagates within the optical fiber 9 almost parallel to the central axis. On the other hand, the light that has propagated within the optical fiber 8 almost parallel to the central axis of the optical fiber is the light ray 13a, 1
3b, the light enters the lens 15 parallel to the optical axis and exits the lens 15 from above the optical axis 01-02 of the lens 15. In this case, as shown by a light ray 13a, a light ray that does not enter the optical axis 01-02 within the lens 15, that is, does not enter the reflective surface 15s, enters the lens 16 as it is. lens 15
By arranging the lenses 16 and 16 with their axes shifted,
This ray will enter the lens 16 at a location away from the optical axis 03-04 of the lens 16, and therefore
The light is converted into a light ray making a large angle with the optical axis, enters the optical fiber 9, and becomes a light ray that propagates within the optical fiber 9 making a large angle with the central axis. On the other hand, among the light that has propagated through the optical fiber 8 at a large angle with the central axis, as represented by the light ray 14, it moves away from the optical axis 03-04 of the lens 16, and therefore on the reflective surface 15s side of the lens 15. The light emitted from the lens 15 enters the reflective surface 15s of the lens 15 and is reflected. As can be easily understood by keeping in mind that the reflected light ray 15s is the meridian plane of the lens 15, it is as if the light emitted from the mirror image position of the optical fiber 8 by the reflective surface 15s has a refractive index distribution shape without the reflective surface 15s. It advances as it advances through the lens, exits from the lens 15, and reaches the optical axis 03-04 of the lens 16.
incident above. The optical axis 03-'04 of this ray is the reflective surface 1.
6s, it is reflected again by the reflective surface 16s,
The light beam is converted by the lens 16 into a light beam substantially parallel to the optical axis 03-04 and enters the optical fiber 9. That is, the light ray 14 is reflected twice by the reflecting surfaces 15s and 16s, so that it almost coincides with the light ray 12 and enters the optical fiber 9. Further, among the light that has propagated through the optical fiber 8 almost parallel to the central axis, the light that is incident on the reflective surface 15s within the lens 15 as shown by the light ray 13b is reflected by this reflective surface and then enters the lens 16. The light is again reflected by the reflective surface 16s and enters the optical fiber 9 at a large angle with the optical axis. As described above, in the mode conversion optical circuit according to the present invention, propagating light beams such as the light beams 12 and 14 emitted from the optical fiber 8, which make a large angle with the central axis of the optical fiber 8, are transferred into the optical fiber 9 through the central axis of the optical fiber 9. Also, as propagating light that forms a small angle with
It has a function of mode-converting propagating light that makes a small angle with the central axis of the optical fiber 8, such as the light rays 13a and 13b, into the propagating light that makes a large angle with the central axis of the optical fiber 9 into the optical fiber 9. Therefore, in the conventional embodiment, appropriate mode conversion was not performed for light represented by the light ray 14, whereas the mode conversion optical circuit according to the present invention has an optimal mode conversion function even for such light rays. It has excellent properties.

As is clear from the above description and FIG. 5, in the mode conversion optical circuit according to the present invention, only one side of the reflective surface of the gradient index lens provided with a reflective surface is actually used. Therefore, it is clear that the gradient index lens passing through the reflective surface may have a semi-cylindrical shape broken at the reflective surface. In addition, in the example, the length of the gradient index lens was set to 10 periods, but the point is that the fiber end face should be placed near the focal plane of the lens, and the length of the lens does not necessarily have to be 10 periods. do not have. As described above, in the mode conversion optical circuit according to the present invention, one
Since two gradient index lenses with one reflecting surface are arranged with their optical axes offset, when this mode conversion optical circuit is inserted between two optical fibers, it is possible to efficiently convert the optical fiber on the input side. The high-order mode components of the light that has been propagated through the optical fiber on the output side are changed to the low-order mode components of the optical fiber on the output side. It has the advantage of being able to perform mode conversion into higher-order mode components.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining light rays propagating within an optical fiber, Figure 2 is a diagram showing an optical fiber transmission line with a mode conversion optical circuit inserted in the middle, and Figure 3 is a cross-sectional view of a conventional mode conversion optical circuit. , FIG. 4 is a diagram showing an embodiment of the mode conversion optical circuit according to the present invention, and FIG. 5 is a sectional view of the embodiment of the present invention shown in FIG. 4.

Claims (1)

[Claims] 1 Length with one of the meridian planes as a reflecting surface is approximately 1/2n+
1/4 period length (n=0, 1, 2,...)
two refractive index distribution shapes, with the meridional planes forming the reflecting surfaces of each lens being approximately parallel to each other, and the optical axes of each lens being approximately perpendicular to the meridian planes forming the reflecting surfaces. The optical fibers are connected in series so that their axes are shifted from each other, and each one optical fiber is located opposite to both ends of the lenses connected in series, and the end surfaces of the optical fibers are formed by the lenses connected in series. What is claimed is: 1. A mode conversion optical circuit characterized in that the mode conversion optical circuit is arranged so as to