JPS6131445B2 - - 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-based optical 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, in which the refractive index is higher toward the center and gradually decreases toward the outer periphery 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 will become a loss, causing a reduction in coupling efficiency. I was getting used to it.

From this point of view, it is an object of the present invention to provide an optical transmission line that results in a high numerical aperture even if a quartz fiber with a low numerical aperture is used.

<Means for Solving the Problems> The optical transmission line of the present invention has one end constituted by a flat surface and the other end constituted by a convex cubic curved surface, and the optical transmission line has a curved surface with a convex three-dimensional shape along the longitudinal direction. The present invention is characterized in that the one end of the numerical aperture converting lens, the refractive index of which is gradually lowered toward the other end, is connected to the end face of the optical fiber.

<Operation> The optical axis that enters the convex lens for numerical aperture conversion while being inclined at a certain angle with respect to the optical axis is bent so that the angle with the optical axis gradually becomes smaller, resulting in a small incidence on the input end face of the optical fiber. incident at the corner.

<Embodiment> An embodiment in which the present invention is applied to a communication optical transmission line will be described in detail with reference to FIGS. 1 and 2.

As shown in FIG. 1, which shows the cross-sectional structure of the entrance end of this embodiment, the entrance end surface 1 has a numerical aperture in the shape of a round bar with a convex three-dimensional curved surface (part of a spherical surface in this embodiment). One end surface of a communication optical fiber 3 is closely connected to the flat exit end surface of the conversion convex lens 2. The refractive index of this numerical aperture converting convex lens 2 in a plane perpendicular to the optical axis 4 is the same at both the center and the periphery, which is different from conventional graded lenses. However, in the axial refractive index distribution, as shown in Figure 2, the refractive index gradually increases from the input end face 1 toward the exit end face connected to the communication optical fiber 3. A light ray 5 that enters the numerical aperture converting convex lens 2 at an incident angle of θ with respect to the optical axis 4 shown by is gradually bent so that the angle with the optical axis 4 becomes smaller, and an exit angle that is significantly smaller than the above-mentioned θ and enters the core portion 6 of the communication optical fiber 3. Therefore, even if the numerical aperture of the communication optical fiber 3 is smaller than the numerical aperture of the entrance end surface 1 side of the numerical aperture conversion convex lens 2, a light beam having an angle of incidence equal to or greater than the maximum angle of incidence defined by the numerical aperture of the communication optical fiber 3 5
It is possible to introduce a large amount of light from the light source compared to the conventional method. In other words, transmission of a strong optical signal can be planned, but the refractive index distribution is not limited to a linear shape as shown in FIG.

In this embodiment, the optical axis 4 of the convex lens 2 for numerical aperture conversion is
Since the refractive index distribution in the direction along the . Therefore, the aperture of the present invention can be achieved by bonding a plurality of glass plates with different refractive indexes in order of refractive index and forming them into a single rod shape, and further bonding a conventional plano-convex lens to the glass plate with the lowest refractive index. It can be used as a convex lens for number conversion. In this case, the refractive index of the plano-convex lens is preferably equal to or lower than that of the glass plate having the lowest refractive index. Note that processing the entrance end surface 1 into a spherical surface can be achieved by softening and melting the entrance end surface 1 using the surface tension of glass. Further, in this embodiment, only the input end was explained, but it is possible to attach the above-mentioned convex lens for numerical aperture conversion to the exit end, and when this is used as an imaging lens of an image fiber, Since the angle of incidence of the light beam that can be coupled to the imaging lens or image fiber increases, bright image transmission can be performed.

<Effects of the Invention> According to the optical transmission line of the present invention, it is possible to manufacture a convex lens for numerical aperture conversion with a refractive index distribution such that the refractive index becomes higher toward the front in the direction of propagation of the light beam, with the same diameter as that of the optical fiber. Although the optical system has the same conditions as a concave lens placed at the rear, the entire system can be made more compact. Moreover, since all the incident light rays converge near the optical axis like an asymptote, it is possible to take in light from a wide angle, and a large amount of light can be transmitted.

[Brief explanation of the drawing]

Fig. 1 is a transmission principle diagram showing the schematic structure of an embodiment in which the present invention is applied to an optical transmission line for communication, and Fig. 2 shows the refractive index distribution along the optical axis direction of the convex lens for numerical aperture conversion used in this. This is a glass to represent the glass, and the symbols in the figure are: 1 is the entrance end surface, 2 is the convex lens for converting the numerical aperture, and 3
4 is an optical fiber for communication, 4 is an optical axis, 5 is a light beam, and 6 is a core portion.

Claims (1)

[Scope of Claims] 1. One end is constituted by a flat surface and the other end is constituted by a convex three-dimensional curved surface, and the refractive index gradually decreases from the one end toward the other end along the longitudinal direction. An optical transmission line characterized in that the one end of a numerical aperture conversion lens having a constant diameter is connected to an end face of an optical fiber. 2. The highest refractive index of a convex lens for numerical aperture conversion, which is formed by bonding a plurality of glass plates each having a different refractive index in the order of the refractive index to form a rod shape, and further bonding a convex lens to the glass plate having the lowest refractive index. 2. The optical transmission line according to claim 1, wherein an end face of an optical fiber is connected to the glass plate having a high resistance.