JPH0264509A - Structure for optical coupling of optical fiber and photodetector - Google Patents

Abstract

PURPOSE:To assure high coupling efficiency, wide band characteristic and low noise characteristic by providing a spherical lens part to the tip of an optical fiber and condensing propagated light through the reflecting film on the surface to a photodetector. CONSTITUTION:The spherical lens 2 is formed to the end of the optical fiber 1 by deviating a center 3 with respect to the optical axis 4. The reflecting film 5 is provided on the surface of this lens. The light propagated in the optical fiber 1 is introduced through the reflecting film 5 to the photodetecting surface 6 of the photodetector 7 by which the light is subjected to opto-electric conversion. The reflecting mirror 5 acts as a concave mirror and the spherical lens 2 acts as a convex lens in such a manner and, therefore, the light is condensed to the min. spot at the proper point of the photodetecting surface 6 parallel with the optical fiber axis 4. The high optical coupling efficiency is thus obtd. even with the small photodetecting surface 6. Since the photodetecting surface 6 can be formed in parallel with the optical axis of the optical fiber 1, the miniaturization of the device is possible.

Description

[Detailed Description of the Invention] Overview Regarding the optical coupling structure of an optical fiber and a light receiving element, an optical coupling structure of an optical fiber and a light receiving element that can ensure high optical coupling efficiency, broadband property, and low noise property and is suitable for miniaturization. For the purpose of providing a coupling structure, a spherical lens part is formed at the end of the optical fiber, the center of this spherical lens part is biased with respect to the axis of the optical fiber, and the surface of the spherical lens part intersects with the axis. A reflective film is formed around the part, and the light propagated through the optical fiber is reflected by the reflective film and taken out from the part of the spherical lens part where the reflective film is not formed, so that the light is received almost parallel to the axis. It is configured so that light is received by a light receiving element having a surface.

INDUSTRIAL APPLICATION FIELD The present invention relates to an optical coupling structure between an optical fiber and a light receiving element.

In recent years, in the field of optical communication or optical transmission, IGb/s
High-speed systems having higher transmission speeds are being actively developed, and as a result, demands for higher speeds in individual system components are increasing. Specifically, an essentially broadband single-mode optical fiber is used as the optical transmission line, and a light-receiving element with a small light-receiving area and small element capacity is used as the light-receiving element. What is required when optically coupling optical fibers and photodetectors, which are becoming increasingly miniaturized, are (a) ensuring high optical coupling efficiency, (b) ensuring broadband and low noise, and (c) ensuring high optical coupling efficiency. ) Suitable for downsizing, etc.

BACKGROUND OF THE INVENTION FIG. 4 is a diagram showing an example of a conventional optical coupling structure between an optical fiber and a light receiving element. The light propagating through the optical fiber 21 is reflected by the obliquely formed end face 21a of the optical fiber, extracted from the side of the optical fiber 21, and the light is converted into electricity by the light receiving element 22. be. 23 is an electronic circuit such as an amplifier for processing the electrical output of the light receiving element 22; 24 is a substrate holding the light receiving element 22 and the electronic circuit 23; In order to reflect the propagating light of the optical fiber 21 at the fiber end face 21a, the end face 21a is formed to have a total reflection angle, or the end face 21a is formed so as to have a total reflection angle.
A reflective film may be formed on 1a.

According to this configuration, since the optical fiber 21 can be held parallel to the substrate 24, it is easy to downsize the device.

FIG. 5 is a diagram showing another conventional optical coupling structure between an optical fiber and a light receiving element. A tapered spherical portion 31a is formed at the end of the optical fiber 31, and the light propagating through the optical fiber 31 is focused by the tapered spherical portion 31a, and is directed to the light receiving element 32 held on the substrate 34 together with the electronic circuit 33. It is designed to perform light-to-electrical conversion. According to this configuration,
Since a light-receiving element with a relatively small light-receiving diameter can be used, it is easy to ensure broadband performance (high-speed operation) and low noise.

Problems to be Solved by the Invention In the conventional example shown in Fig. 4, the way the emitted light spreads is different between the optical axis direction of the optical fiber (X direction) and the X direction perpendicular to the X direction and parallel to the substrate surface. , since the emitted light is not focused in any direction, the light receiving surface 22H of the light receiving element 22
As shown in Fig. 6, the irradiation pattern becomes larger depending on the distance between the optical fiber and the light receiving element (
In particular, it spreads greatly in the X direction. ). Therefore, in order to ensure high optical coupling efficiency, a light-receiving element with a large light-receiving area and large element capacity is required, which hinders broadband performance and low noise performance. In order to avoid this, it may be proposed to bring the optical fiber close to the light receiving element, but this may cause damage to the light receiving surface. On the other hand, if a small-diameter light-receiving element is used with the optical fibers separated, although broadband performance is ensured, the optical coupling efficiency decreases.

In the conventional example shown in Figure 5, it is possible to ensure high optical coupling efficiency, broadband performance, and low noise, but it is difficult to miniaturize the device because the optical fiber and the board are arranged vertically. There is a problem that.

The present invention was created in view of these circumstances, and provides an optical coupling structure between an optical fiber and a light receiving element that can ensure high optical coupling efficiency, broadband performance, and low noise, and is suitable for miniaturization. It is an object.

Means to solve problems FIG. 1 is a diagram showing the principle of the present invention.

1 is an optical fiber, 2 is a spherical lens part formed at the end of the optical fiber 1, and the center 3 of this spherical lens part 2 is
It is biased with respect to the axis 4 of the optical fiber 1.

Reference numeral 5 denotes a reflective film formed around a portion of the surface of the spherical lens portion 2 that intersects with the axis 4.

Then, the light propagating through the optical fiber 1 is reflected by the reflective film 5 and taken out from the part of the spherical lens part 2 where the reflective film 5 is not formed, and the light receiving element having the light receiving surface 6 substantially parallel to the axis 4 is formed. The light is received at 7.

Function According to the configuration of the present invention, the light propagated through the optical fiber 1 is reflected by the reflective film 5 which acts as a concave mirror and is emitted from the side of the spherical lens part 2 which acts as a convex lens. The irradiation surface of the emitted light in a plane parallel to the axis of the optical fiber is minimized at an appropriate location. Therefore, even if the emitted light is received by a light-receiving element having a small light-receiving surface, high optical coupling efficiency can be obtained, and therefore, high optical coupling efficiency, broadband performance, and low noise can be ensured. Further, since the light receiving surface of the light receiving element is substantially parallel to the axis of the optical fiber, miniaturization is easy. In addition, a configuration in which positive refractive power is imparted only to the outgoing light extraction portion of the spherical lens portion, that is, a portion where a reflective film is to be formed is formed on a flat surface, and this flat surface satisfies the total reflection condition. When comparing the configuration of the present invention with the configuration of the present invention, the present invention has a higher light focusing ability due to the reflective film functioning as a concave mirror, and the shape of the spherical lens portion and the amount of deviation of its center point from the fiber axis. , and the degree of freedom regarding the arrangement of the light-receiving surface increases.

In the specification of this application, a spherical lens portion does not necessarily mean a lens having a true spherical shape, and does not mean that the portion on which the reflective film is formed functions as a concave mirror, and the portion that extracts the emitted light functions as a convex lens. It means something that works.

Example Embodiments of the present invention will be described below based on the drawings.

FIG. 2 is a partially cutaway side view of an optical receiver showing an embodiment of the present invention. Reference numeral 11 denotes an optical fiber, and a tapered portion 12 is formed at the end of the optical fiber, the diameter of which decreases toward the end. Reference numeral 13 denotes a spherical lens section provided at the tip of the tapered section 12, the center of which is deviated with respect to the axis of the optical fiber 11. A reflective film 14 made of a metal, dielectric multilayer film, or the like having a sufficiently high reflectance at the wavelength of the light used is formed at least on the irradiated portion of the optical fiber 11 that is opened in the spherical lens portion.

16 is a light-receiving element such as an APD (avalanche photodiode), and its light-receiving surface 15 has a broadband property (up to several Gb/
s) has a sufficiently small area (for example, 50 μm or less in diameter) to ensure low noise. Light receiving element 1
6 is fixed on a substrate 17 together with an electronic circuit 18 such as an amplifier, and these light receiving elements 16 and electronic circuits 18 are interconnected by bonding wires 19.

According to such a configuration, the light emitted from the spherical lens B13 can be focused to a spot size sufficiently smaller than 50 μm in diameter, so by arranging the light receiving surface 15 of the light receiving element near the focusing point, , high optical coupling efficiency can be obtained. In this case, the light source is relatively It is possible to eliminate the influence of reflected feedback light on the light source when the light source is at a short distance. On the other hand, under the condition that the light receiving surface 15 of the light receiving element is arranged near the convergence point of the light emitted from the spherical lens part 13, the optical fiber 11 is fixed to the substrate 17 via a suitable holding means. In this case, optical fiber 11 and substrate 1
7 are substantially parallel, making it possible to easily downsize the optical receiver. Furthermore, in this embodiment, since the light receiving element 16 and the electronic circuit 18 can be brought sufficiently close to each other on the substrate 17, their electrical interconnection can be performed over an extremely short distance, resulting in broadband performance and low cost. It is easy to ensure noise resistance.

Hereinafter, a method of processing the tip of an optical fiber will be explained with reference to FIG. First, as shown in FIG. 4A, the optical fiber 11 is stretched while being heated by arc discharge, a burner such as an H202 burner, or other heating means, and after cooling, the optical fiber 11 is cut at the constricted portion. Next, when the optical fiber 11 is vertically supported so that the cut surface is located downward and heated from below with a burner, the taper of the optical fiber formed by stretching is shown in FIG. Since the tip of the portion 12a is partially melted, surface tension is generated in the melted portion and a minute spherical portion 13a is formed. If the heating of the tip is continued, the spherical part will increase and the tapered part will shrink, as shown in FIG. 13 can be formed. The taper ratio of the tapered portion 12, that is, the rate of change in the diameter in the longitudinal direction of the fiber, can be controlled by the amount of stretching of the optical fiber. Then, as shown in FIG. 3(d), when the optical fiber 11 is supported horizontally and its end is heated and softened by a low-temperature burner, the center 0 of the spherical lens portion 13 is aligned with the optical fiber due to the action of gravity. It can be biased toward the optical axis ○A of No. 11. If the spherical lens part 13 is small and it is difficult to deform it only by the action of gravity, it may be deformed by the ejection force of an inert gas such as nitrogen or the ejection force of a burner flame, or by using a solid material such as a platinum rod. The deformation may be performed by bringing the two into contact with each other. Finally, as shown in FIG. 4E, a reflective film 14 is partially formed on the surface of the spherical lens portion 130 by, for example, vapor deposition.

The reason why the tapered portion is formed in this embodiment is as follows. In other words, if a tapered part is not formed, the radius of curvature of the spherical lens part cannot be made smaller than the radius of the optical fiber, but if a tapered part is formed as in this example, the radius of curvature of the spherical lens part cannot be made smaller than the radius of the optical fiber. This is because a large light condensing effect can be obtained by making the radius smaller than the radius of the optical fiber. The present invention relates to an optical coupling structure between an optical fiber and a light-receiving element, but if we focus on the fact that the optical path is reversible with respect to reflection and refraction of light, the present invention can be applied to an optical coupling structure between an optical fiber and a light-emitting element. It is easy to use it for physical bonding structures.

Effects of the Invention As detailed above, according to the present invention, high optical coupling efficiency and high optical coupling efficiency can be achieved when optically coupling an optical fiber and a light receiving element.
This has the effect of ensuring wide band performance and low noise, and making it possible to downsize the device.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the principle of the present invention, Fig. 2 is a partially cutaway side view of an optical receiver showing an embodiment of the present invention, and Fig. 3 is a method for processing the tip of an optical fiber used in carrying out the present invention. 4 is a diagram showing a conventional optical coupling structure between an optical fiber and a light receiving element; FIG. 5 is a diagram showing another example of a conventional optical coupling structure between an optical fiber and a light receiving element; FIG. 6 is a diagram for explaining problems in the conventional example shown in FIG. 4. 1.1 2, 1 5.1 6.1 7.1 1... Optical fiber, 3... Spherical lens portion, 4... Reflective film, 5... Light receiving surface, 6... Light receiving element .

Claims (2)

[Claims] (1) A spherical lens part (2) is formed at the end of the optical fiber (1), and the center (3) of this spherical lens part (2) is biased with respect to the axis (4) of the optical fiber (1). A reflective film (5) is formed around a portion of the surface of the spherical lens portion (2) that intersects with the axis (4), and the light propagating through the optical fiber (1) is reflected by the reflective film (5).
5) to reflect the reflection film (5) of the spherical lens part (2).
) is removed from the part where the shaft (4) is not formed.
1. An optical coupling structure of an optical fiber and a light receiving element, characterized in that light is received by a light receiving element (7) having a light receiving surface (6) substantially parallel to ). (2) The optical fiber and light receiving element optical coupling structure according to claim 1, further comprising a tapered portion whose diameter decreases as it approaches the end of the optical fiber.