JPS6131444B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a high numerical aperture optical transmission line, and is particularly suitable for an optical transmission line using a quartz fiber.

<Conventional technology> In recent years, with the development of optical fibers, instead of conventional optical lenses, graded round rod-shaped lenses have been introduced, which have a higher refractive index toward the center and gradually lower toward the outer circumference in the radial direction. Lenses (rod lenses) have been created, and because they can be made much more compact than optical lenses, they are used as imaging optical systems and condensing lenses that are connected to the terminals of optical fibers.

<Problems to be solved by the invention> Silica fiber, which is said to have the lowest transmission loss, has a smaller numerical aperture than other fibers, that is, a narrow range of light incident angles, so it is difficult to use for communication signals. When transmitting light, only a small portion of the light from the light source can be transmitted and a powerful light source is required. Furthermore, when this quartz fiber is used as an image fiber for image transmission, it is not possible to transmit a bright image because the numerical aperture is small. Furthermore, if the numerical apertures of these optical systems differ at the joints between the optical systems, the incident angle of the light ray will be determined by the optical system with the smallest numerical aperture, so even if a graded lens with a large numerical aperture is used. Even when used on the light introduction side, if the numerical aperture of the optical fiber itself is small, a portion of the light that enters the optical fiber from this graded lens becomes a loss, which causes a reduction in coupling efficiency. It was becoming.

From this point of view, an object of the present invention is to provide an optical transmission line with a high numerical aperture even when using an optical fiber with a low numerical aperture.

<Means for Solving the Problems> The optical transmission line of the present invention has a refractive index that gradually decreases toward the outer periphery in the radial direction with respect to the refractive index of the center, and a refractive index that decreases in the optical axis direction with respect to the refractive index of one end surface. The refractive index on the other end surface side along the direction parallel to is lower, and the refractive index difference between the center and the radial outer peripheral side on the other end surface side is lower than the refractive index difference between the center and the radial outer peripheral side on the one end surface side. The present invention is characterized in that the one end surface of the graded lens having a larger refractive index difference is connected to the end surface of an optical fiber.

<Function> The light beam that is incident on the other end surface of the graded lens in an inclined state has its amplitude gradually attenuated and does not leak to the outside, and enters the incident end surface of the optical fiber at a smaller angle of incidence. .

<Example> An example of the optical transmission line according to the present invention will be described in detail with reference to FIG. 1, which shows the general principle of the input end.

One end surface of a graded lens 1 whose refractive index gradually decreases toward the outer periphery in the radial direction from the center is bonded to the end surface of a single mode transmission optical fiber 2 in a butt state. The refractive index distribution of this graded lens 1 is always higher on one end surface side than on the other end surface side where the light ray 4 is incident, in the direction parallel to the optical axis 3 shown by the dashed line in the figure. However, the refractive index difference Δn between the center and the outer peripheral side in the radial direction is larger on the other end surface side than on the one end surface side. In other words, the numerical aperture on the other end surface side is larger than the numerical aperture on the one end surface side, and the refractive index distribution along the radial direction of the graded lens 1 in this example is shown in FIG. The upper 〓 line in the middle represents the refractive index distribution at one end surface, and the lower 〓
The physical line represents the refractive index distribution at the other end surface. Note that the refractive index distribution in the direction parallel to the optical axis 3 is
As mentioned above, it is in a state where it continuously decreases in a straight line from one end surface to the other end surface. Therefore, the light ray 4 incident from the other end surface of the graded lens 1 at an incident angle of θ ₂ has its amplitude gradually attenuated, and a single mode emerges from the one end surface at an exit angle of θ _{1, which is smaller than θ 2 .} The light enters the core portion 5 of the transmission optical fiber 2. In this case, since the refractive index is higher toward the front in the traveling direction of the light ray 4, the light ray 4 traveling while bending inside the graded lens 1 does not seep out to the outside at all, and the numerical aperture of the other end surface is defined. All of the light rays incident on the graded lens 1 at the maximum angle of incidence will be sent into the core portion 5 of the single mode transmission optical fiber 2. Therefore, this graded lens 1 functions as an extremely efficient condensing lens, and is capable of guiding a strong optical signal into the core portion 5.
In this embodiment, the input end of the single mode optical fiber has been described, but the graded lens may also be attached to the exit end of the single mode optical fiber.

Next, another embodiment of the optical transmission line according to the present invention will be described with reference to FIGS. 3 and 4.

As shown in FIG. 3, which shows the general principle of this embodiment, in this embodiment, a plurality of (four in the figure) graded lenses 11, 12, 1 with different numerical apertures are used.
3 and 14 are connected in order of numerical aperture, and the graded lens 11 with the smallest numerical aperture is connected to the end face of the image fiber 16. Therefore, in this embodiment, four graded lenses 11, 12,
13 and 14 are combined to form one graded lens (imaging lens) 15, which is used for image transmission. This embodiment uses the same graded lens 1 as the conventional one, unlike the graded lens 1 described above. In other words, although there is no change in the refractive index distribution in the optical axis direction, as shown in FIG. The graded lens 14 having the largest numerical aperture has the lowest refractive index. In the figure, a lens having a refractive index difference of Δn ₂ corresponds to a graded lens 12, and a lens having a refractive index difference of Δn ₃ corresponds to a graded lens 13. Manufacturing the graded lens 1 by continuously changing the refractive index in the optical axis direction as in the above-described embodiment means that, for example, in the vapor phase axial method, the amount of dopant supplied and its distribution can be changed over time. Although it is a relatively troublesome manufacturing procedure as the lens must be changed, in this embodiment, different types of graded lenses 11, 12, 1 are used.
3 and 14, and it is also relatively easy to control the difference in numerical aperture between both ends. From this point of view, the above-mentioned graded lens 1
It is also possible to join a plurality of them to form one piece as in this embodiment.

In this embodiment, since the refractive index of the incident light ray 17 increases in the direction in which it travels, it hardly bleeds outward, and gradually converges in the center and enters the image fiber 16 at a small exit angle. . Therefore, it is possible to transmit a bright image even if the image fiber 16 is made of a fiber with a small numerical aperture. By connecting the above-mentioned graded lens to the exit end of the image fiber 16, the brightness can be set arbitrarily.

<Effects of the Invention> According to the optical transmission line of the present invention, while changing the numerical aperture of both end faces of a graded lens, it is possible to prevent light rays incident from the end face with a large numerical aperture from leaking out to the outside on the way, resulting in increased loss. Since the refractive index is higher toward the front in the direction of light ray propagation, even if a silica fiber with a small numerical aperture is used, it can actually be used as one with a large numerical aperture, and therefore when applied to optical transmission lines for communication. It is possible to introduce a strong optical signal into an optical fiber, and when applied to an image transmission line, a bright image can be transmitted.

[Brief explanation of the drawing]

Fig. 1 is a geometric principle diagram showing the principle of propagation of light rays at the input end in an embodiment in which the present invention is applied to a communication optical transmission line, and Fig. 2 is a radial refractive index distribution on both end surfaces of the graded lens. 3 is a geometrical principle diagram showing the principle of propagation of light rays at the incident end in an embodiment in which the present invention is applied to an image transmission path, and FIG. 4 is a graph showing the four graded lenses constituting the graded lens. It is a graph showing the radial refractive index distribution of a shaped lens. In the symbols in the figure, 1 is a graded lens, 2
is an optical fiber for single mode transmission, 4 is a light beam, 1
1, 12, 13, and 14 are conventional graded lenses; 15 is a graded lens obtained by combining conventional graded lenses; 16 is an image fiber; and 17 is an incident light beam.

Claims (1)

[Claims] 1 The refractive index of the center becomes gradually lower as the outer peripheral side in the radial direction becomes lower, and the refractive index of the other end face along the direction parallel to the optical axis direction becomes lower than the refractive index of one end face, and The graded lens has a constant diameter, and the refractive index difference between the center and the radial outer circumferential side on the front other end surface side is larger than the refractive index difference between the center and the radial outer circumferential side on the one end surface side. An optical transmission line characterized in that one end face is connected to the end face of an optical fiber.