Light-receiving structure for waveguide type optical devices

LIGHT-RECEIVING STRUCTURE FOR WAVEGUIDE-TYPE OPTICAL DEVICES

The present invention relates to a light-receiving structure for waveguidetype optical devices, and more particularly to a light-receiving structure for waveguide-type optical devices applicable to optical-fibre communication.

Waveguide-type optical devices are suitable for the integration of such 5 functions as light branching and coupling and optical-demultiplexing and multiplexing. They also permit mass-production in a batch process. For these reasons, great hope is pinned on their application to optical transmitter/receiver modules for use in subscriber optical communication systems. Such optical transmitter/receiver modules usually have built-in light-receiving elements together with elements for transmitting use, such as semiconductor laser elements. These transmitting and receiving elements are arranged on a substrate. Therefore, it is necessary to efficiently couple the light emitted from the optical waveguide to the light-receiving element.

Light-receiving structures for waveguide-type optical devices according to the prior art include what is disclosed in Japanese Laid-open Patent No. 036508/1991 (Reference 1). However, the light-receiving structure described in Reference 1 has its groove formed with a dicing saw, and is thus subject to serious damage on the optical waveguide. It does not provide for its end face being mirror-ground, resulting in the problem that the propagating light suffers heavy loss. There is involved another problem, namely that because a 45degree inclination is needed, the productivity of groove formation is poor.

Another example of the prior art is the light-receiving structure disclosed in Japanese Laid-open Patent No. 183605/1989 (Reference 2). In this structure, a light-receiving element is mounted, with the light-receiving face upward, on a circuit substrate provided in parallel to an optical fibre. The end face of the optical fibre adheres tightly to one of the end faces of the optical waveguide substrate while the other end face of the optical waveguide is inclined. A lens is glued to the under face of the optical waveguide opposite to this inclined end face. The signal light having travelled over the optical fibre comes incident on

B the optical waveguide and, after being reflected by the inclined end face, is coupled by the lens to the light-receiving element. The light-receiving structure described in Reference 2 has the disadvantage that, because it consists of many constituent elements, it cannot be built compactly, and is inconvenient and costly to manufacture.

An object of the present invention is to realize a light-receiving structure for waveguide-type optical devices, excelling in productivity, compactly formed and permitting highly-efficient coupling.

In a light-receiving structure for waveguide-type optical devices according to one aspect of the present invention there is a waveguide substrate on which an optical waveguide is formed and a light-receiving element to be opticallycoupled to an incident light coming from the optical waveguide. The lightreceiving element is mounted, with its light receiving face upward, on the bottom face of a recess which is lower than the surface of the waveguide substrate. The optical waveguide has an end face near the light-receiving element, and the light emitted from the end face is reflected by a reflector arranged on the light-receiving element and thereby coupled to the light-receiving element.

According to another aspect of the invention, in a light-receiving structure for waveguide-type optical devices, there is a waveguide substrate in which an optical waveguide is formed and a light-receiving element to be optically-coupled to an incident light coming from the optical waveguide. The optical waveguide has a through-hole on the light path on which the optical waveguide is formed, and the light-receiving element is mounted, with its light-receiving face directed toward the through-hole, on the face opposite to that on which the optical waveguide of the waveguide substrate is formed. The optical waveguide has an end face near the through-hole, and the light emitted from the end face is reflected by a reflector arranged on the light-receiving element and thereby coupled to the light receiving element.

The above-mentioned and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein:

Figure 1 shows a cross-sectional view of a light-receiving device according to the prior art;

Figure 2 shows a cross-sectional view of a light-receiving structure for waveguide-type optical devices according to a first preferred embodiment of the invention;

Figure 3 shows a cross-sectional view of a light-receiving structure for waveguide-type optical devices according to a second preferred embodiment of the invention;

Figure 4 shows a cross-sectional view of a light-receiving structure for 10 waveguide-type optical devices according to a third preferred embodiment of the invention;

Figure 5 shows a perspective view of the internal structure of the lightreceiving structure for waveguide-type optical devices according to the third preferred embodiment of the invention;

Figure 6 illustrates an example of the application of a light-receiving structure for waveguide-type optical devices to a bidirectional transmitter/receiver optical module;

Figure 7 shows an expanded view of the light-receiving structure for waveguide-type optical devices according to the embodiment of Figure 6; and,

Figure 8 shows an expanded view of the light-receiving structure of the third embodiment of the invention, used in place of the light-receiving structure for waveguide-type optical devices according to the embodiment of the invention illustrated in Figure 7.

With a view to facilitating the understanding of the present invention, the configuration of a light-receiving structure for waveguide-type optical devices according to the prior art will be described first. Figure 1 shows a crosssectional view of a light-receiving device described in the aforementioned Reference 1. A silicon dioxide waveguide 32 cladded with silicon dioxide layers 32a and 32b is formed on a silicon substrate 31, and a light-receiving element

33 is mounted on it. A groove is formed, for instance with a dicing saw, from the upper face of the silicon substrate 31 so that the cut face form an inclination

B ) of 45 degrees against the propagating light axis of an optical waveguide 2. A signal light having travelled over the optical waveguide 32 is reflected upward by the end face of the groove, and becomes incident on the light-receiving element 33 to be coupled to it. Such a structure, as stated previously, requires the formation of a groove in the substrate, is subject to serious damage on the optical waveguide and permits no satisfactory coupling. Furthermore, the reflective face, which is flat, cannot condense light and accordingly is incapable of highly-efficient coupling.

Next, Figure 2 is used to describe a first preferred embodiment in detail.

Figure 2 shows a cross-sectional view of a light-receiving structure for waveguide-type optical devices according to a first preferred embodiment of the invention. An optical waveguide 2 is formed on a first silicon substrate 1. Part of the first silicon substrate 1 has a recess 7 which is slightly lower than the surface of the first silicon substrate 1, and a light-receiving element 3 is mounted in the recess 7 with its light-receiving face upward. A PIN photo-diode chip or an avalanche photo-diode chip is preferably used as a light-receiving element 3. On the other hand, the optical waveguide 2 has an end face 8 formed near the recess 7.

Further over the first silicon substrate 1 is formed a second silicon substrate 4, which is provided with a groove 6 positioned opposite to the optical waveguide 2 and a reflector 5 positioned opposite to the light-receiving face of the light-receiving element 3.

A signal light becomes incident on the optical waveguide 2 from an outside source, such as a transmission path optical fibre. After the incident signal light passes functional elements (not shown) for optical branching, optical demultiplexing and the like, all formed on the substrate 1, a light is emitted from the end face 8. The emitted light first expands, is reflected by the reflector 5 of the second silicon substrate 4, and received by the light-receiving face of the light-receiving element 3.

Next will be described a second preferred embodiment of the present invention with reference to Figure 3.

The light-receiving structure for waveguide-type optical devices according to the invention illustrated in Figure 3 features a concave reflector, which serves to reduce the reflection loss due to radiation. The shape of the concave reflector 15 can be so determined that the light emitted from an end face 18 of an optical waveguide 12 is reflected and condensed to a diameter either equal to or slightly smaller than the light-receiving diameter of a light-receiving element 13.

A reflector cannot be formed in a concave shape by anisotropic optical etching of the silicon substrate as in the case of the flat reflector of the first embodiment. In this case, if a cover 14 is made of mold resin, covers each having a reflector 15 in any desired concave shape can be mass-produced.

Still another preferred embodiment of the present invention will be described now with reference to Figure 4.

Figure 4 shows a cross-sectional view of a third preferred embodiment of the invention. A silicon substrate 21 has a through-hole 29, and a lightreceiving element 23 is mounted with its light-receiving face directed toward the through-hole 29. A cover provided with a concave reflector 25 is arranged over the silicon substrate 21 in a position opposite to the through-hole 29. As in the second embodiment, a light reflected by the concave reflector 25 is condensed and coupled to the light-receiving element 23.

If the light-receiving element is mounted on the same face where an optical waveguide is formed as in the first and second embodiments, there may be problems in that the effective area of the substrate surface is limited by the recess, the electrode wiring to the light-receiving element should pass through the recess and, if any part of the electrode wiring crosses the optical waveguide, the process will become complicated. The limitations are particularly great in high-density integration. By contrast, the structure of the third embodiment, where the light-receiving element is mounted on the underface, is free from the aforementioned limitations.

In every embodiment, the coupling can be further strengthened by coating the reflector surface with a film of gold or some other substrate with a high reflection factor.

Next will be described the first preferred embodiment in more specific detail.

On the face of the first silicon substrate 1 in the crystal face orientation (100) is formed a mask pattern of silicon dioxide by photolithography. Subsequent anisotropic chemical etching with, for instance, an aqueous solution of potassium hydroxide as etchant results in the formation of a recess having an inclined side face in the crystal face orientation (111). Above this recess is formed the silicon-dioxide-based optical guide 2, cladded with silicon dioxide, doped with titanium dioxide and having silicon dioxide as a core. On the bottom of the recess is mounted the light-receiving element 3 via an insulating layer of silicon dioxide formed, for instance, by thermal oxidation. A conductor pattern is provided on the silicon substrate 1 to achieve electric connection between the light-receiving element 3 and an external lead terminal.

In the second silicon substrate 4 is also formed a recess by anisotropic etching as in the first silicon substrate 1. A side face of the crystal face orientation (111), inclined by 54.74 degrees against the crystal face (100), is fixed above the first silicon substrate 1, with its top and bottom inverted, in a position opposite to the optical waveguide 2 on the first silicon substrate 1. On the inclined face of the second silicon substrate 4 is vapor-deposited a film of gold over a chromium undercoat to increase the reflection factor.

On the junction sides of the first and second silicon substrates 1 and 4 are pre-formed metallic film patterns for matching the positions of soldering bumps, and they are soldered together after these patterns are matched with each other.

This light-receiving structure enables the signal light, after having travelled the optical waveguide 2, and after being emitted from the end face 8, to be reflected by the inclined reflective face 5 of the second silicon substrate 4, and to arrive incident on the light-receiving face of the light-receiving element

3. The description of this specific embodiment applies to the second and third embodiments as well.

Next will be described an embodiment in which a light-receiving structure for waveguide-type optical devices according to the present invention is applied to an optical transmitter/receiver module for bidirectional optical transmission. Figure 6 shows a perspective view of the transmitter/receiver optical module for bidirectional optical transmission, in which an optical waveguide 32 having a silicon dioxide core is formed on a silicon substrate 31. One end of the optical waveguide 32 is connected to an optical fibre 38, which is further connected to a transmission path optical fibre. In the silicon substrate 31 is formed in advance a V-groove 42, in which the optical fibre 38 is arranged. At this time, the core of the optical fibre 38 and the optical waveguide 32 become coincident in position and are optically-coupled.

Midway on the optical waveguide 32 is formed a Y-branching part for twoway branching. The terminal end of one branched optical waveguide 32b is optically-coupled to a semiconductor laser 43 arranged on the surface of the substrate 31. An optical signal emitted from the semiconductor laser 43 travels over the optical waveguide 32b, and is coupled to the optical fibre 38 to be sent out to the transmission path.

At the terminal end of the other branched optical waveguide 32a from the Y-branching of the optical waveguide 32 is arranged a reflector cover 34, which optically-couples the light emitted from the optical waveguide 32a to the lightreceiving element 33.

Next, for ready understanding of the light-receiving structure for waveguide-type optical devices according to the invention in this particular embodiment, the following description will refer to Figure 7. Figure 7 shows an expanded view of the light-receiving structure. The silicon substrate 31 is provided with a recess 37, formed in advance by chemical etching. On this recess 37 is mounted the light-receiving element 33 consisting of a germanium avalanche photodiode, with its light-receiving face 33a upward.

On the other hand, the reflector cover 34 is made of plastic and has a concave reflector 35 inside. This reflector cover 34 is formed by molding, and the surface of the concave reflector 35 is coated with films of chromium and gold to increase its reflection factor. The mounting face (not shown) of the reflector cover 34 is similarly coated. Furthermore, one side face of the reflector cover 34 is provided with a groove 36 to pass the optical waveguide 32a within.

The reflector cover 34 is arranged so as to substantially coincide with the metallic film-part formed around the recess 37 on the substrate 31, and fixed with soldering bumps. The fixation with soldering bumps makes it possible to determine the relative positions of the optical waveguide 32, the reflector 35 and the light-receiving face 33a accurately. Wiring from the light-receiving element to any external circuit is accomplished by a metallic film pattern (not shown) provided on the substrate 31.

Next will be described, with reference to Figure 8, a case in which the third preferred embodiment of the present invention is applied to the abovedescribed optical transmitter/receiver module. The light-receiving structure illustrated in Figure 8 differs from that shown in Figure 7 in that the lightreceiving element is arranged on the underside of the substrate 31, but is the same in all other respects.

The recess 37 has a through-hole 39. In this embodiment, the throughhole 39 is conventionally bored by irradiation with an excimer laser. To the underface of the substrate 31 is fastened the light-receiving element 33, with its light-receiving face directed toward the through-hole 39. A received-light emitted from the optical waveguide 32a is reflected by the concave reflector 35, passes through the through-hole 39, and irradiates the light-receiving face 33a. It should be noted that, since the concave reflector 35 condenses the emitted light even if the distance is long from the end face of the optical waveguide 32a to the light-receiving face 33a via the reflector 35, the light can be coupled to the light-receiving element 33 at high efficiency.

In the waveguide-type optical devices illustrated in Figures 7 and 8, since the semiconductor laser 43 and its drive circuit are arranged close to each other on the same substrate, the light-receiving element 33 is susceptible to electric noise from outside. In view of this, the external face of the reflector cover 34 is conventionally coated with a brass film 40, which readily cuts off the noise g

which would otherwise reach the light-receiving element 33, and thereby facilitates high-quality reception.

To compare the light-receiving structures according to the invention illustrated in Figures 7 and 8, that shown in Figure 7 is superior in that the packaged structure is simpler. However, in the structure shown in Figure 8, the light-receiving element 33 is arranged on the underside of the substrate 31, and its light-receiving circuits and the similar circuits can also be arranged on the underface. Therefore, this structure is superior in that the effect of electric noise from the transmitting side, arranged on the substrate 31, can be more effectively avoided.

As hitherto described, the light-receiving structure for waveguide-type optical waveguides according to the present invention enables the optical waveguide and the light-receiving element to be efficiently coupled to each other by forming a recess in part of the surface of the substrate on which the optical waveguide is formed, mounting the light-receiving element on the recess with its light-receiving face upward and arranging a cover, in which a reflector is formed, in a position opposite to the light-receiving element.

The cover in which the reflector is provided permits mass production by using a silicon substrate and applying anisotropic chemical etching. It is also possible to form the concave shape of the reflector by fabricating the cover by molding resin. This helps to reduce the coupling loss, and also permits mass production of the cover.

Since the cover is mounted simply by being brought-over and fixed to the substrate, it allows ready assembling. Furthermore, by boring a through-hole in the substrate in advance and mounting the light-receiving element with its light-receiving face toward the through-hole, it becomes possible to mount the light-receiving element on the underface of the substrate for increased packaging density.

Thus, the present invention makes it possible to realize a light-receiving structure for waveguide-type optical devices, which excels in productivity, is compactly-formed and permits highly-efficient coupling.