1. Field of the Invention
The present invention relates to a photocoupler used for optically coupling between an optical waveguide type optical pick-up device and an optical waveguide device, and a method for producing the same.
2. Description of the Related Art
Conventionally, a photocoupler provided with a prism for introducing incident light to an optical waveguide having no tapered structure is generally known.
Recently, there have been demands for an optical waveguide type optical pick-up device with an even smaller beam spot diameter for realizing recording/reproduction of higher density. However, a photocoupler mounted on an optical waveguide type optical pick-up device has a problem that an allowable incident position area with respect to a coupling efficiency becomes smaller as an incident beam diameter becomes smaller (i.e., a beam spot diameter of 30 .mu.m or less). This problem has to be solved in order to meet the demands of smaller beam spot diameter.
A prism coupler shown in FIG. 20 is disclosed in Japanese Laid-Open Publication No. 4-289531 for solving the above-described problem. In the above-mentioned prism coupler, an allowable incident position area with respect to coupling characteristics is enlarged by varying a propagation constant (a value having a real part representing a phase constant and an imaginary part representing an attenuation constant, which is determined by a film thickness and a refractive index of the optical waveguide) along a direction of light propagation of the optical waveguide. Specifically, the above-mentioned prism coupler employs a second gap layer 104 with a tapered structure shown in FIG. 20.
The prism coupler includes a substrate (Si substrate) 101, an SiO.sub.2 layer 107, an optical waveguide layer 102, a first gap layer 103 which acts as an air gap and the second gap layer 104 with the tapered structure provided in this order. A prism 106 is bonded to the tapered second gap layer 104 via a bonding section (adhesive) 105.
FIG. 21 is a cross-sectional view showing a prism coupler including a prism 304 which is in pressure contact with an optical waveguide 305. The optical waveguide 305 includes a substrate 303, a first dielectric layer 301 and a second dielectric layer 302. An edge (edge surface) 306 of the prism 304 has the same function as the second gap layer 104 shown in FIG. 20.
A photocoupler such as a prism coupler exhibits the highest efficiency for incident light that matches a radiation pattern of outgoing light from the photocoupler. The actual radiation pattern (i.e., an intensity distribution of the outgoing light from the photocoupler) is difficult to analyze. Therefore, herein, a "radiation pattern" is assumed to be approximated by an amount of change in the amplitude of the propagating light in every microscopic section .DELTA.x traced in a certain direction (e.g., in a propagation direction).
Therefore, a radiation pattern of outgoing light from the prism coupler shown in FIG. 20 is controlled to have a desired shape by tapering the second gap layer 104. Specifically, the second gap layer 104 is tapered (with a gentle slope of 1:1000 or more) such that the prism coupler almost uniformly outputs light with a uniform intensity in a direction of propagation of the optical waveguide (in other words, a radiation pattern has a shape such that the intensity thereof is generally uniform regardless of variations in the incident positions), thereby enhancing the coupling efficiency for the incident beam having a relatively large beam diameter.
Another conventional prism coupler is disclosed in Japanese Laid-Open Publication No. 5-45532 and is shown in FIG. 22. This prism coupler is provided with a tapered optical waveguide layer (guiding layer).
Although the specification of the Japanese Laid-Open Publication No. 5-45532 does not describe about enhancing the coupling characteristics of the prism coupler, there is a possibility that an allowable incident position area with respect to the coupling characteristics for incident light may be enlarged in the prism coupler shown in FIG. 22.
The optical waveguide of the prism coupler shown in FIG. 22 includes a substrate 201 and an optical waveguide layer 202 having a tapered portion formed thereon. A prism 204 having a gap layer 205 formed on the bottom surface 206 thereof (which will make direct contact with the optical waveguide) is bonded to the optical waveguide with an adhesive 203 having a refractive index generally equal to that of the optical waveguide layer 202.
As described above, in the prism coupler shown in FIG. 20, the radiation pattern is intentionally changed by partially tapering the second gap layer 104 into a gentle slope. In the case where the prism coupler shown in FIG. 20 is used as a photocoupler for a light beam with a small beam diameter (while adjusting the slope of the tapered portion), a distance between an edge 111 of the bonding section 105 and an edge 110 of the tapered portion (i.e., a boundary between a tapered portion and a flat portion) of the second gap layer 104 will have an influence on the coupling characteristics to some degree.
Since the prism coupler shown in FIG. 20 has no factor of defining the edge 111 of the bonding section 105 to make contact with the top surface of the optical waveguide in a straight line, the distance between the edge 111 of the bonding section 105 and the edge 110 of the tapered portion of the second gap layer 104 cannot be accurately positioned in order to optimize the coupling efficiency. Accordingly, the prism coupler shown in FIG. 20 has a problem that when it is used for a light beam with a small beam diameter, it is difficult to enlarge the allowable incident position area with respect to the coupling efficiency while restraining reduction in the coupling efficiency.
Furthermore, the thickness of the waveguide device needs to be increased for tapering the second gap layer 104 in order to obtain a desired radiation pattern of outgoing light from the prism coupler. Accordingly, freedom of the device design may be limited or a membrane stress may be increased which disturbs precise production of the prism coupler.
On the other hand, in the case of the prism coupler shown in FIG. 22, simply providing the optical waveguide layer 202 with a tapered structure does not enlarge the allowable incident position area with respect to the coupling characteristics for the following reason.
A photocoupler exhibits a higher coupling efficiency for incident light with a phase and an intensity distribution closer to the radiation characteristics (the radiation pattern) of outgoing light from the photocoupler which is in use. The tapered structure of the optical waveguide layer 202 will produce either a monotonous decreasing type radiation pattern or a varied type radiation pattern, both shown in FIG. 23. The monotonous decreasing type radiation pattern is also obtained when the optical waveguide layer is not tapered. The monotonous decreasing type radiation pattern undesirably allows the coupling efficiency to be easily changed in accordance with the change in the incident position of light after the light is coupled to the optical waveguide in the propagation direction of the light. Therefore, an allowable incident position area in the case of the monotonous decreasing type radiation pattern would be smaller than that of the varied type radiation pattern. In order to enlarge the allowable incident position area with respect to the coupling characteristics, the photocoupler preferably maintains the intensity of the radiation pattern to be constant in spite of the changes in the incident position. Accordingly, the varied type radiation pattern having a maximum point in the midway with gentle curves declining on both sides is preferable.
However, since the tapered portion of the prism coupler shown in FIG. 22 is not defined taking the above-described point in consideration, there is a problem that the allowable incident position area with respect to the coupling efficiency cannot be increased.
In addition, in the case of the prism coupler shown in FIG. 22, the prism 204 with the gap layer 205 is bonded to the optical waveguide with the adhesive 203 having the same refractive index as the optical waveguide layer 202. As a result, when an alignment error is caused between the prism 204 having the gap layer 205 and the optical waveguide, the alignment error will proportionally result in an offset from an optimum relative positions (for obtaining the maximum coupling efficiency) between the edge 208 of the prism 204 and the edge 207 of the tapered portion of the optical waveguide layer 202.
Moreover, in the case of the photocoupler shown in FIG. 22, since the thickness of the bonding section 203 is difficult to control and the straightness thereof is not guaranteed, it is difficult to optimize the distance between the edge 208 of the prism 204 and the edge 207 of the tapered portion of the optical waveguide layer 202. As a result, it is difficult to enlarge the allowable incident position area with respect to the coupling efficiency while restricting the reduction in the coupling efficiency.
Thus, conventionally, prism couplers have a problem of not being able to accommodate a smaller beam spot diameter which is required for realizing recording/reproduction with higher density.
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