Optical waveguide module

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical waveguide module having a planar waveguide type of optical waveguide formed on a substrate.

[0004] 2. Related Background Art

[0005] In an optical circuit that makes use of an optical waveguide, such as an optical fiber or planar optical waveguide, it is sometimes desirable to control the optical intensity of signal light to a suitable value, such as by keeping the optical intensity constant for signal light transmitted through optical waveguides. In such a case, the optical intensity of the signal light is monitored in the optical circuit, and sometimes the optical intensity is also controlled on the basis of the results of this monitoring.

SUMMARY OF THE INVENTION

[0006] This monitoring of the optical intensity of signal light used to be accomplished by providing an optical coupler on an optical waveguide and splitting part of the signal light. With this method, an optical coupler is provided at a specific location along the optical waveguide, a few percent of the signal light is split, and the optical intensity of this split light is monitored with a photodetector, which allows the optical intensity of the signal light transmitted through this optical waveguide to be monitored.

[0007] A problem with this use of an optical coupler, though, is that it increases the number of optical components that make up the optical circuit, and furthermore, since these have to be fused together, the structure and manufacturing process of the optical circuit become more complicated.

[0008] In contrast, methods have been proposed in which an optical coupler is not used, and instead part of the signal light is reflected and the optical intensity monitored. For instance, with the optical device discussed in Japanese Patent Application Laid-Open H6-331837, an end face is formed at an angle with respect to the optical axis at a specific location along an optical waveguide, reflected light, part of the signal light reflected from this end face in a direction different from the optical axis, is detected, and the optical intensity is monitored. Also, the optical fiber discussed in Japanese Patent Application Laid-Open 2000-155235 relates to a light splitting/merging structure, in which an end face is formed perpendicular to the optical axis at a specific location along an optical fiber, part of the signal light is emitted outside, and part of the emitted light is reflected and taken off by another end face that is inclined with respect to the optical axis.

[0009] However, when part of the signal light is thus reflected and used to monitor the optical intensity, the reflectivity of the signal light at the inclined end face varies with the state of polarization of the signal light being reflected. Consequently, a problem is that the optical intensity cannot be correctly monitored unless the state of polarization of the signal light transmitted through the optical waveguide is specified. Also, the end face of the optical waveguide can become contaminated if it is exposed to the outside atmosphere, resulting in inferior long-term stability of the reflectivity and so forth.

[0010] The present invention was conceived in an effort to solve the above problems, and it is an object thereof to provide an optical waveguide module with which the structure of the optical circuit is simpler, and the optical intensity can be correctly monitored regardless of the state of polarization of the signal light.

[0011] To achieve the stated object, the optical waveguide module according to the present invention is characterized by comprising (1) a planar waveguide optical circuit, consisting of a substrate and a planar waveguide type of optical waveguide formed on this substrate, and having a groove formed at a predetermined inclination angle θ (0°<θ) with respect to the vertical axis perpendicular to the optical axis of the optical waveguide so as to cross a specific area of the optical waveguide, (2) a reflection filter that is installed on the inside of the groove of the planar waveguide optical circuit including a portion where signal light transmitted through the optical waveguide passes through, and that reflects part of the signal light according to a specific reflectivity in which the difference in reflectivity between the orthogonal polarization has been compensated, (3) a filler resin installed so as to seal at least the inside of the groove, and (4) a photodetector that detects reflected light of the signal light reflected by the reflection filter.

[0012] With the above-mentioned optical waveguide module, rather than the optical waveguide being split with an optical coupler, of the signal light is reflected in an inclined groove provided to the optical waveguide, and the optical intensity of the signal light can be monitored by means of this reflected light. As a result, the structure and manufacturing process of the optical circuit are simplified.

[0013] Also, rather than the reflection of the signal light being performed on the end face of the groove, a reflection filter that has undergone polarization compensation for equalizing the reflectivity between the polarization states is installed in the groove, and part of the signal light is reflected by this reflection filter and used to monitor optical intensity. Here, since the reflectivity of the signal light by the reflection filter becomes substantially constant regardless of the polarization state of the signal light transmitted through the optical waveguide, it is possible to monitor the optical intensity accurately regardless of the polarization state of the signal light. Also, since the inside of the groove including the reflection filter is sealed with a filler resin, the end face of the groove and the reflection filter do not come into contact with the outside atmosphere, which prevents any deterioration in long-term stability that would otherwise be caused by contamination of the end face and so forth.

[0014] The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.

[0015] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]
FIG. 1 is a plan view of the structure in a first embodiment of the optical waveguide module;

[0017]
FIG. 2 is a cross section and detail enlargement of the structure of the optical waveguide module shown in FIG. 1, cut along the optical axis;

[0018]
FIG. 3 is a graph of the change in polarization dependent loss when the inclination of the reflection surface is varied with respect to the optical axis along which the signal light is transmitted;

[0019]
FIGS. 4A to 4D are schematic views illustrating the shape of the reflected light spot produced by the reflection filter, and the shape of the light receiving area of a photodetector;

[0020]
FIG. 5 is a plan view of the structure in the second embodiment of the optical waveguide module;

[0021]
FIG. 6 is a cross section and detail enlargement of the structure of the optical waveguide module shown in FIG. 5, cut along the optical axis;

[0022]
FIG. 7 is a plan view of the structure in the third embodiment of the optical waveguide module;

[0023]
FIG. 8 is a cross section and detail enlargement of the structure of the optical waveguide module shown in FIG. 7, cut along the I-I line;

[0024]
FIG. 9 is a plan view of the structure in the fourth embodiment of the optical waveguide module;

[0025]
FIG. 10 is a cross section and detail enlargement of the structure of the optical waveguide module shown in FIG. 9, cut along the II-II line;

[0026]
FIG. 11 is a plan view of the structure in the fifth embodiment of the optical waveguide module; and

[0027]
FIG. 12 is a cross section and detail enlargement of the structure of the optical waveguide module shown in FIG. 11, cut along the III-III line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Preferred embodiments of the optical waveguide module according to the present invention will now be described in detail through reference to the drawings. Those elements that are the same will be numbered the same in the description of the drawings, and redundant descriptions will be omitted. The scale in the drawings does not necessarily coincide with that in the description.

[0029]
FIG. 1 is a plan view of the structure in a first embodiment of the optical waveguide module according to the present invention. This optical waveguide module comprises a planar waveguide optical circuit 1 having a substrate 10 and eight (eight channels) planar waveguide type of optical waveguides 21 to 28 formed on the substrate 10.

[0030] The optical waveguides 21 to 28 are formed parallel to each other and equidistantly, extending from an input end 11 of the planar waveguide optical circuit 1 to an output end 12 in a predetermined optical transmission direction (the direction indicated by the arrow in FIG. 1). A groove 3 that cuts across the optical waveguides 21 to 28 is provided at a specific location with respect to the optical transmission direction of the planar waveguide optical circuit 1.

[0031] On the inside of this groove 3 is installed a reflection filter 4 for reflecting part of the signal light transmitted through the optical waveguides 21 to 28 respectively, with the reflection filter 4 being sealed in the groove 3 by a filler resin 5. A sub-mounting substrate 7 is installed above the planar waveguide optical circuit 1 at a location further upstream than the groove 3, and a photodetector array 6 having eight photodetectors 61 to 68 corresponding to the optical waveguides 21 to 28 is installed above the filler resin 5 and the sub-mounting substrate 7. The planar waveguide optical circuit 1 and the sub-mounting substrate 7 are soldered together, for example, as are the sub-mounting substrate 7 and the photodetector array 6.

[0032] In FIG. 1, the dotted lines indicate the shapes of the light receiving areas of the photodetectors 61 to 68. The sub-mounting substrate 7 is a mounting member for mounting the photodetector array 6, and on its top surface are formed wiring, electrodes, etc., for reading out the photodetection signals from the photodetectors 61 to 68, as shown schematically in FIG. 1.

[0033]
FIG. 2 is a cross section of the structure of the optical waveguide module shown in FIG. 1, cut along the optical axis (the optical transmission direction of the planar waveguide optical circuit 1) of the optical waveguides 2n (n=1 to 8). In FIG. 2, the portion including the groove 3, reflection filter 4, and the photodetector array 6 is shown enlarged.

[0034] As shown in FIG. 2, the optical waveguide 2n in the planar waveguide optical circuit 1 comprises lower cladding 22, a core 20, and upper cladding 21 formed on the substrate 10. The groove 3 that cuts across the optical waveguide 2n at a specific area is formed at a specific inclination angle θ (0°<θ) with respect to the vertical axis (shown by the dotted line in FIG. 2) perpendicular to the optical axis of the optical waveguide 2n (perpendicular to the substrate 10), to a depth d including at least the area where the signal light transmitted through the optical waveguide 2n passes through, corresponding to the core 20. In this embodiment, the depth d of the groove 3 is set to be greater than the thickness of the optical waveguide 2n.

[0035] The reflection filter 4 is inserted on the inside of the groove 3. The reflection filter 4 is installed so as to include at least the area where the signal light transmitted through the optical waveguide 2n passes through, at substantially the same angle θ as the groove 3 with respect to the optical axis. This reflection filter 4 preferably consists of a dielectric multilayer film filter, and is designed such that part of the signal light with a specific wavelength (within the specified wavelength band) transmitted through the optical waveguide 2n is reflected at a predetermined reflectivity.

[0036] Furthermore, the dielectric multilayer film filter that constitutes this reflection filter 4 is formed so that the difference in reflectivity is compensated for between the orthogonal polarization when signal light is reflected, and the signal light components in various states of polarization are reflected at substantially the same reflectivity. This setting of the reflectivity with respect to the signal light components in various polarization states is accomplished, for example, by varying the dielectric materials of the respective layers that make up the dielectric multilayer film filter, the combinations of these material, the thickness of the respective layer films, etc.

[0037] The inside of the groove 3, including the reflection filter 4, is sealed with the filler resin 5. The filler resin 5 used in this embodiment is composed of an inner filler resin 51 that seals the inside of the groove 3, and an upper filler resin 52 that seals a specific portion of the top surface of the planar waveguide optical circuit 1 including the upper part of the groove 3. The inner filler resin 51 and upper filler resin 52 are integrally formed using the same resin material.

[0038] The photodetector array 6, which has photodetectors 6n (n=1 to 8) corresponding to the optical waveguide 2n, is installed on the top side of the sub-mounting substrate 7 and the upper filler resin 52 part of the filler resin 5. The photodetector array 6 is disposed such that the reflected light obtained when part of the signal light transmitted through the optical waveguide 2n is reflected by the reflection filter 4 will be incident on the light receiving area of the corresponding photodetector 6n. Because of the incident direction of the reflected light, it is preferable for the photodetectors 6n to be a rear incidence type of photodiode or the like.

[0039] With the above structure, when the signal light transmitted through an optical waveguide 2n is emitted through an upstream end face 31 to the inner filler resin S inside the groove 3, the reflection filter 4, which is inclined with respect to the optical axis, reflects part of this signal light diagonally above the planar waveguide optical circuit 1 at a predetermined reflectivity that has undergone polarization compensation so as to be equal with respect to polarization states. The rest of the signal light passes through the inner filler resin 51 and the reflection filter 4, and is again incident on the optical waveguide 2n via a downstream end face 32.

[0040] Meanwhile, the light reflected by the reflection filter 4 is incident on the photodetector 6n via the inner filler resin 51, the optical waveguide 2n, and the upper filler resin 52. The optical intensity of the signal light transmitted through the optical waveguide 2n is monitored on the basis of the optical intensity of the reflected light detected by the photodetector 6n.

[0041] With the optical waveguide module in this embodiment, rather than the optical waveguide itself being split by means of an optical coupler or other such optical components, part of the signal light is reflected in the inclined groove 3 provided so as to cut across the respective optical waveguides 2n, and used for monitoring the optical intensity. This simplifies the structure of the optical circuit of an optical waveguide module with which the optical intensity can be monitored. Also, there is no need to install extra optical components, fuse the optical waveguides together, and so forth, so the manufacturing process is similarly simplified.

[0042] The reflection of the signal light is not performed by the end faces 31 and 32 of the groove 3, and instead part of the signal light is reflected by the reflection filter 4, which has undergone polarization compensation so that the reflectivity between the orthogonal polarization will be substantially equal. Here, since the reflectivity of the signal light produced by the reflection filter 4 becomes substantially constant regardless of the polarization state of the signal light transmitted through the optical waveguide 2n, the optical intensity of the reflected light detected by the photodetector 6n can be used to accurately monitor the optical intensity regardless of the polarization state of the signal light.

[0043] Also, the inside of the groove 3 including the reflection filter 4 is sealed by the filler resin 5. Since the end faces 31 and 32 of the groove 3, the reflection filter 4, and so forth do not come into contact with the outside atmosphere, any deterioration in long-term stability that would otherwise be caused by contamination of these components is prevented.

[0044] It is preferable here if the inclination angle θ of the groove 3 and the reflection filter 4 with respect to the vertical axis is within an angle range of 0°<θ≦40°.

[0045]
FIG. 3 shows the change in polarization dependent loss (PDL) when we vary the inclination of the reflection surface with respect to the optical axis along which the signal light is transmitted. As shown in this graph, the reflection characteristics with an ordinary reflection surface are such that there is no polarization dependence and PDL=0 when the inclination angle θ with respect to the vertical axis is 0° (when the signal light is reflected in the opposite direction). As the inclination angle θ increases, so does the polarization dependence of reflectivity, and there is a sharp increase in the PDL value once θ goes over 40°.

[0046] If θ thus becomes larger and the PDL increases, it becomes difficult for the reflection filter 4 to perform polarization compensation for the difference in reflectivity. Specifically, with a structure in which the inclination angle θ is large, the PDL value is large and varies sharply with θ, so extremely stringent numerical conditions are required for the refractive index of the respective layer materials, the film thickness, and so forth in the dielectric multilayer film filter used to compensate for the difference in reflectivity. Consequently, for practical purposes it is difficult to design and produce the dielectric multilayer film filter of the reflection filter 4 so that the difference in reflectivity will be sufficiently compensated for. In contrast, if the inclination angle θ is set to an angle range of 0°<θ≦40°, then the difference in reflectivity can be compensated for at an adequate precision at the reflection filter 4.

[0047] It is preferable for the filler resin 5 to be composed of a resin material having substantially the same refractive index as the core 20 of the optical waveguides 2n (such as within a margin of error of 1%).

[0048] If the inner filler resin 51 is a resin material having substantially the same refractive index as the core 20, then unnecessary reflection will be minimized when signal light is emitted from the optical waveguides 2n to the inner filler resin 51 (see point P1 in FIG. 2). Also, unnecessary reflection will be minimized when the light reflected by the reflection filter 4 is incident from the inner filler resin 51 to the optical waveguides 2n (see point P2).

[0049] If the upper filler resin 52 is a resin material having substantially the same refractive index as the core 20, then unnecessary reflection will be minimized when the light reflected by the reflection filter 4 is incident from the optical waveguide 2n to the upper filler resin 52 (see point P3).

[0050] If unnecessary reflection occurs at the interfaces of the optical waveguides 2n, the inner filler resin 51, and the upper filler resin 52, the reflection characteristics of these components will have polarization dependence according to the respective reflection angles thereof. Therefore, if reflection occurs without the refractive index being matched between these components, then even though polarization compensation is performed for the difference in reflectivity at the reflection filter 4, the optical intensity of the reflected light detected by the photodetector 6n will depend on the polarization state of the signal light. In contrast, if the refractive indexes are matched so as to minimize unnecessary reflection at the respective interfaces, it will be possible to accurately monitor optical intensity regardless of the polarization state of the signal light.

[0051] When the upper filler resin 52 is provided in addition to the inner filler resin 51 as in this embodiment, it is preferable to use resin materials having substantially the same refractive index for these filler resins 51 and 52. This allows the unnecessary reflection of signal light (reflected light) at the interface of the inner filler resin 51, as well as the unnecessary reflection of signal light (reflected light) at the interface of the upper filler resin 52 to be similarly controlled.

[0052] The filler resins 51 and 52 may also be formed integrally, using the same resin material. This simplifies the step of installing the resin. The difference in refractive index between the core 20 and the upper cladding 21 is usually small enough to be ignored for the purposes of this reflection problem.

[0053] If a problem is posed by unnecessary reflection when the light reflected by the reflection filter 4 is incident from the upper filler resin 52 to a photodetector 6n (see point P4), it is preferable to provide an antireflective coating at the interface between the filler resin 5 and the photodetector 6n. This minimizes the unnecessary reflection that occurs when reflected light from the reflection filter 4 is incident on the photodetector 6n, and makes it possible to accurately monitor optical intensity regardless of the polarization state of the signal light, just as with the unnecessary reflection at the above-mentioned interfaces of the optical waveguides 2n, the inner filler resin 51, and the upper filler resin 52.

[0054] As to the structure of the photodetectors 6n and the photodetector array 6, as shown in FIG. 1, it is preferable if the light receiving areas of the photodetectors 6n are formed in a substantially elliptical shape whose major axis lies in the direction of the optical axis of the optical waveguides 2n, and whose minor axis lies in the direction in which the optical waveguides 2n are laid out.

[0055]
FIGS. 4A to 4D are schematic views illustrating the shape of the reflected light spot of the reflected light produced by the reflection filter 4, and the shape of the light receiving area of a photodetector 6n. The signal light transmitted through an optical waveguide 2n has a substantially circular signal light spot because of the shape of the core 20, etc. Therefore, the reflected light produced when this signal light is reflected by the inclined reflection filter 4 (see the side view in FIG. 4A) forms an elliptical reflected light spot at the light receiving area of the photodetector 6n, as indicated by the dotted line A in the plan view of FIG. 4B.

[0056] On the other hand, if the light receiving areas of the photodetectors 6n are formed in a substantially elliptical shape corresponding to the elliptical shape of the reflected light spot and encompassing this reflected light spot, as indicated by the solid line B in FIG. 4B, the reflected light can be detected at a sufficiently high light receiving efficiency. It is also possible to use a lens or the like to focus the elliptical reflected light before it is incident on the photodetector, but this drives up the cost because it entails more optical components. By contrast, if the shape of the light receiving areas of the photodetectors 6n is made elliptical to match the reflected light spot, the structure of the optical waveguide module will be simplified and the cost reduced.

[0057] If the shape of the light receiving area is circular, as indicated by the solid line C in FIG. 4C, the efficiency at which the reflected light is received will be the same as with a substantially elliptical shape, but the light receiving area will be wider in the direction in which the photodetectors are laid out, and which corresponds to the vertical direction in FIG. 4C (see 61 to 68 in FIG. 1). Consequently, the photodetectors cannot be brought together at as high a density or as small a layout pitch in the photodetector array, which increases the surface area of the optical circuit and drives up its cost.

[0058] By contrast, if the shape of the light receiving area is substantially elliptical, the layout pitch of the photodetectors can be kept to a minimum, affording a more efficiency optical circuit configuration.

[0059] If the shape of the light receiving area is rectangular, as indicated by the solid line D in FIG. 4D, the photodetectors can be grouped together at the same layout pitch as with a substantially elliptical shape, but this configuration results in an unnecessary light receiving area portion that is not used to receive the reflected light being produced at the corners of the light receiving area. This light receiving area portion becomes a source of noise with respect to the photodetection signal, and therefore results in a decrease in efficiency in reflected light detection, such as a deterioration in S/N ratio or a smaller effective dynamic range. The same problem is encountered with the above-mentioned circular light receiving area.

[0060] On the other hand, if the shape of the light receiving area is substantially elliptical, this will reduce the generation of noise and so forth from the unnecessary light receiving area portion, allowing the optical intensity to be monitored more efficiently. Still, a light receiving area that is circular or rectangular may be used as dictated by the layout pitch, detection efficiency, and so on required of an individual optical waveguide module.

[0061] Specific examples will now be given for the optical waveguide module according to the above embodiment.

[0062] A first example will now be given. In this example, the groove 3 was formed at an inclination angle θ of 30° and a width w of 25 μm in the optical axis direction. The reflection filter 4 inserted inside the groove 3 was a polarization compensated filter with a width of 11 μm and a 10% reflectivity of signal light. The filler resin 5 comprised the same resin material for both the inner filler resin 51 and the upper filler resin 52. The refractive index of the resin material, an adhesive with adjusted refractive index, was n=1.47 at a wavelength band of 1.51 to 1.61 μm. The photodetectors 6n were each an InGaAs-PIN photodiode having an elliptical light receiving area with a diameter of 0.3 mm in the major axis direction and a diameter of 0.15 mm in the minor axis direction. The photosensitivity of this photodiode alone was 1.1 A/W.

[0063] The optical intensity of signal light was monitored using an optical waveguide module configured as above, whereupon the polarization dependence of reflectivity at the reflection filter 4 was 10% for S polarization and 10.3% for P polarization, and the polarization dependent loss (PDL) was sufficiently low at 0.1 dB, which confirmed that it was possible to monitor the optical intensity in a state in which dependence on the polarization state of the signal light has been sufficiently reduced.

[0064] The photosensitivity with respect to the incident signal light was about 0.1 A/W. This indicates that the optical intensity of the signal light component reflected by the reflection filter 4 at a reflectivity of 10% was detected by the photodetectors 6n at an efficiency of close to 100%. The insertion loss of the groove 3 and the reflection filter 4 into the optical waveguides 2n was about 1.0 dB, including the loss produced by reflection of the signal light by the reflection filter 4, diffraction of the signal light inside the groove 3, and so forth.

[0065] A second example will now be given. In this example, the groove 3 was formed at an inclination angle θ of 10° and a width w of 25 μm in the optical axis direction. The reflection filter 4 inserted inside the groove 3 was a polarization compensated filter with a width of 11 μm and a 10% reflectivity of signal light. The filler resin 5 comprised the same resin material for both the inner filler resin 51 and the upper filler resin 52. The refractive index of the resin material, an adhesive with adjusted refractive index, was n=1.47 at a wavelength band of 1.51 to 1.61 μm. The photodetectors 6n were each a photodiode having an elliptical light receiving area with a diameter of 0.3 mm in the major axis direction and a diameter of 0.15 mm in the minor axis direction. The photosensitivity of this photodiode alone was 1.1 A/W.

[0066] In this example, an antireflective coating was provided at the interface between the upper filler resin 52 and the photodetectors 6n in order to prevent unnecessary reflection that would be produced if reflected light were incident on the photodetectors 6n.

[0067] The optical intensity of signal light was monitored using an optical waveguide module configured as above, whereupon the polarization dependence of reflectivity at the reflection filter 4 was 9.7% for S polarization and 10% for P polarization, and the polarization dependent loss (PDL) was sufficiently low at 0.1 dB, which confirmed that it was possible to monitor the optical intensity in a state in which dependence on the polarization state of the signal light has been sufficiently reduced.

[0068] The photosensitivity with respect to the incident signal light was about 0.1 A/W. This indicates that the optical intensity of the signal light component reflected by the reflection filter 4 at a reflectivity of 10% was detected by the photodetectors 6n at an efficiency of close to 100%. The insertion loss of the groove 3 and the reflection filter 4 into the optical waveguides 2n was about 1.0 dB, including the loss produced by reflection of the signal light by the reflection filter 4, diffraction of the signal light inside the groove 3, and so forth.

[0069] It can be seen from the first and seconds examples given above that optical waveguide modules with the above configurations make it possible to accurately monitor optical intensity regardless of the polarization state of the signal light.

[0070] Another embodiment of the optical waveguide module according to the present invention will now be described.

[0071]
FIG. 5 is a plan view of the structure in the second embodiment of the optical waveguide module. Just as with the first embodiment, this optical waveguide module comprises a planar waveguide optical circuit 1 having a substrate 10 and eight (eight channels) planar waveguide type of optical waveguides 21 to 28 formed on the substrate 10.

[0072] The optical waveguides 21 to 28 are formed parallel to each other and equidistantly, extending from an input end 11 of the planar waveguide optical circuit 1 to an output end 12 in a predetermined optical transmission direction (the direction indicated by the arrow in FIG. 5). A groove 3 that cuts across the optical waveguides 21 to 28 is provided at a specific location with respect to the optical transmission direction of the planar waveguide optical circuit 1.

[0073] On the inside of this groove 3 is installed a reflection filter 4 for reflecting part of the signal light transmitted through the respective optical waveguides 21 to 28, with the reflection filter 4 being sealed in the groove 3 by a filler resin 5. A photodetector array 60 having eight photodetectors 611 to 618 corresponding to the optical waveguides 21 to 28 is installed above the planar waveguide optical circuit 1 at a location further upstream than the groove 3. In FIG. 5, the dotted lines indicate the shapes of the light receiving areas of the photodetectors 611 to 618.

[0074]
FIG. 6 is a cross section of the structure of the optical waveguide module shown in FIG. 5, cut along the optical axis of the optical waveguides 2n (n=1 to 8) In FIG. 6, the portion including the groove 3, the reflection filter 4, and the photodetector array 60 is shown enlarged. In the configuration shown in FIG. 6, the reflection filter 4, the groove 3, and the planar waveguide optical circuit 1 including the optical waveguides 2n comprising the lower cladding 22, the core 20, and the upper cladding 21 are configured the same as in FIG. 2.

[0075] The inside of the groove 3, including the reflection filter 4, is sealed with the filler resin 5. The filler resin 5 used in this embodiment is composed of only the inner filler resin 51 that seals the inside of the groove 3.

[0076] The photodetector array 60, which has photodetectors 61n (n=1 to 8) corresponding to the optical waveguide 2n, is installed on the top side of the upper cladding 21 of the planar waveguide optical circuit 1. The photodetector array 60 is disposed such that the reflected light obtained when part of the signal light transmitted through the optical waveguide 2n is reflected by the reflection filter 4 will be incident on the light receiving area of the corresponding photodetector 61n. Because of the incident direction of the reflected light, it is preferable for the photodetectors 61n to be a rear incidence type of photodiode or the like. An antireflective coating may also be provided at the interface between the photodetectors 61n and the upper cladding 21 of the optical waveguides 2n.

[0077] With the above structure, when the signal light transmitted through an optical waveguide 2n is emitted through the upstream end face 31 to the inner filler resin 51 inside the groove 3, the reflection filter 4, which is inclined with respect to the optical axis, reflects part of this signal light diagonally above the planar waveguide optical circuit 1 at a specific reflectivity that has undergone polarization compensation so as to be equal with respect to respective polarization states. The rest of the signal light passes through the inner filler resin 51 and the reflection filter 4, and is again incident on the optical waveguide 2n via the downstream end face 32.

[0078] Meanwhile, the light reflected by the reflection filter 4 is incident on the photodetector 61n via the inner filler resin 51 and the optical waveguide 2n. The optical intensity of the signal light transmitted through the optical waveguide 2n is monitored on the basis of the optical intensity of the reflected light detected by the photodetector 61n.

[0079] With the optical waveguide module in this embodiment, just as with the first embodiment, part of the signal light is reflected in the inclined groove 3 provided so as to cut across the respective optical waveguides 2n, and used for monitoring the optical intensity. This simplifies the structure of the optical circuit of an optical waveguide module with which the optical intensity can be monitored. Also, there is no need to install extra optical components, fuse the optical waveguides together, and so forth, so the manufacturing process is similarly simplified.

[0080] Also, since part of the signal light is reflected by the reflection filter 4, which has undergone polarization compensation so that the reflectivity between the orthogonal polarization will be substantially equal, the optical intensity of the reflected light detected by the photodetector 61n can be used to accurately monitor the optical intensity regardless of the polarization state of the signal light. Further, since the inside of the groove 3 including the reflection filter 4 is sealed by the filler resin 5, the end faces 31 and 32 of the groove 3, the reflection filter 4, and so forth do not come into contact with the outside atmosphere, any deterioration in long-term stability that would otherwise be caused by contamination of these components is prevented.

[0081] If, as in the above first and second embodiments, the planar waveguide optical circuit 1 has N number (N is a plurality; N=8 in the above embodiments) of optical waveguides and the optical intensity is monitored for N channels of signal light transmitted through these respective optical waveguides, then N number of photodetectors are installed corresponding to the respective optical waveguides. The N channels of signal light reflected by the reflection filter will propagate through the respective reflected light paths and be detected by the corresponding photodetectors, and as a result the optical intensity of each channel of signal light will be monitored.

[0082] Here, the signal light from the optical waveguides reflected by the reflection filter is detected by the corresponding photodetectors, but in some cases light scattering or reflection produced at various locations of the device, the spreading of the transmitted light, and so forth may cause part of the light to be incident on photodetectors of other adjacent channels, resulting in crosstalk. If crosstalk thus occurs between adjacent channels, the optical intensity of the signal light of those channels cannot be monitored accurately.

[0083] There are a number of possible causes of crosstalk between channels. For instance, with the structure shown in FIG. 2 or 6, possible causes of crosstalk include scattered reflection at the interface between the filler resin 5 and the various parts of the planar waveguide optical circuit 1, scattering caused by defects at the top surface of the upper cladding 21, etc., and reflected light and scattered light caused by scattered reflection at the interface between the substrate 10 and the lower cladding 22.

[0084] The signal light transmitted through an optical waveguide mainly propagates through the core, but part of the signal light spreads out and also propagates through the upper and lower cladding near the core. If there is too much spreading of the signal light to the cladding, the reflected light path from the reflection filter to the photodetector can spread out excessively, unnecessary scattered light can be generated within the optical waveguide or elsewhere, and so on, all of which can cause crosstalk.

[0085] This spreading of the signal light to the cladding also occurs, for example, as a result of misalignment of the optical fibers used for inputting the signal light, connected to the input end of the planar waveguide optical circuit, with respect to the optical waveguides. Specifically, if an optical fiber is connected at a location away from the core of the optical waveguide, the inputted signal light will propagate through the cladding and other components that do not have a waveguide structure.

[0086] In the second embodiment, in which this problem of crosstalk between channels was handled by directly installing the photodetector array 60 on the top side of the planar waveguide optical circuit 1, without providing a sub-mounting substrate or the like, the reflected light path from the reflection filter 4 to the photodetector 61n is shorter and the spreading of reflected light can be reduced, as shown in FIG. 6. Also reduced is the effect of scattered reflection or scattering at the interface or on the inside of the optical waveguides 2n or filler resin 5. Therefore, crosstalk between the channels is suppressed.

[0087] Also, enhancing the confinement of the signal light within the core 20 with respect to the signal light transmitted through the optical waveguides 2n is preferable in terms of preventing crosstalk in both the first and second embodiments. In specific terms, the refractive index differential Δn between the core 20 and the cladding 21 and 22 should be increased. This decreases the spreading of the signal light transmitted through the core 20 into the cladding 21 and 22. Because of the high refractive index of the core, an effect of confining the light inside the core will also be achieved for the signal light component that leaks into the cladding due to misalignment of an optical fiber, etc.

[0088] As an example, with the structure shown in FIG. 6, when an optical fiber was connected in a state in which the fiber was misaligned by being offset by 2 μm from the core of an optical waveguide with a core size of 8.5 μm square, and a refractive index differential Δn of 0.3% between the core 20 and the cladding 21 and 22, crosstalk worsened to −22 dB. In contrast, when the refractive index differential Δn was raised from 0.3% to 0.45%, crosstalk improved to a level of <−25 dB.

[0089] We will now describe a method for manufacturing a planar waveguide optical circuit when the refractive index differential Δn between the core and cladding is thus increased. The core and the upper cladding (overcladding) are formed, for example, by using an SiO2 (quartz) glass micropowder to which the respective required additives have been added, depositing this powder by flame hydrolysis deposition (FHD) method, and sintering.

[0090] More specifically, the core can be germanium(Ge)-added SiO2 glass, and the upper cladding can be boron/phosphorus(B/P)-added SiO2 glass, for example. The germanium, boron, phosphorus, and SiO2 are obtained in the form of soot-like micropowders by hydrolyzing GeCl4, BCl3, POCl3, and SiCl4, respectively, in an oxyhydrogen burner. The refractive index differential Δn is adjusted by adjusting the amount of germanium added to the core and the amounts of boron and phosphorus added to the upper cladding.

[0091] For instance, the amount of germanium added to the core is typically about 3.2 wt % Ge concentration when Δn=0.3%, and about 4.6 wt % Ge concentration when Δn=0.45%. The amounts of boron and phosphorus added to the upper cladding are determined by the sooting conditions, etc., but are adjusted so that the amount of added phosphorus, which increases the refractive index, will be balanced with the amount of added boron, which decreases the refractive index, and the refractive index will be equivalent to that of pure SiO2 glass. The above method yields optical waveguides with a refractive index differential Δn between the core and the upper cladding of 0.3% and 0.45%.

[0092] In addition to enhancing the effect of confining the light within the core, providing a light path isolation means between the light paths is also an effective way to deal with the above-mentioned problem of crosstalk between channels. Specifically, providing light path isolation means between adjacent reflected light paths almost completely prevents crosstalk from occurring between adjacent channels. As a result, the optical intensity of the N channels of signal light transmitted through the respective optical waveguides can be monitored more accurately.

[0093]
FIG. 7 is a plan view of the structure in the third embodiment of the optical waveguide module. Just as with the first embodiment, this optical waveguide module comprises a planar waveguide optical circuit 1 having a substrate 10 and eight (eight channels) planar waveguide type of optical waveguides 21 to 28 formed on the substrate 10.

[0094] The optical waveguides 21 to 28 are formed parallel to each other and equidistantly, extending from an input end 11 of the planar waveguide optical circuit 1 to an output end 12 in a predetermined optical transmission direction (the direction indicated by the arrow in FIG. 7). A groove 3 that cuts across the optical waveguides 21 to 28 is provided at a specific location with respect to the optical transmission direction of the planar waveguide optical circuit 1.

[0095] On the inside of this groove 3 is installed a reflection filter 4 for reflecting part of the signal light transmitted through the respective optical waveguides 21 to 28, with the reflection filter 4 being sealed in the groove 3 by a filler resin 5. A photodetector array 62 having eight photodetectors 631 to 638 corresponding to the optical waveguides 21 to 28 is installed above the planar waveguide optical circuit 1 at a location further upstream than the groove 3.

[0096] In FIG. 7, one-dot chain lines indicate the photodetector array 62 and the photodetectors 631 to 638 in order to illustrate the positional relationship between the photodetector array 62 and the various components, and to show the planar waveguide optical circuit 1 in a state in which the photodetector array 62 has been removed.

[0097] In this embodiment, a light path isolation means for isolating reflected light paths is provided within the planar waveguide optical circuit 1 in between each of the eight reflected light paths along which part of the signal light transmitted through the eight channels of optical waveguides 21 to 28 is reflected by the reflection filter 4 to the corresponding photodetectors 631 to 638. This light path isolation means serves to suppress the generation of crosstalk between adjacent channels.

[0098] In specific terms, in this embodiment a light blocking layer 25 is provided between each of the eight optical waveguides 21 to 28 so as to block light from passing from one reflected light path into an adjacent reflected light path within the planar waveguide optical circuit 1.

[0099] Specifically, light blocking layers 251, 252, 253, 254, 255, 256, and 257 are respectively provided between the optical waveguides 21 and 22, between the optical waveguides 22 and 23, between the optical waveguides 23 and 24, between the optical waveguides 24 and 25, between the optical waveguides 25 and 26, between the optical waveguides 26 and 27, and between the optical waveguides 27 and 28.

[0100] Other than the above-mentioned light blocking layers 251 to 257 being provided to the planar waveguide optical circuit 1, the structure of the optical waveguide module in this third embodiment is the same as that in the second embodiment shown in FIGS. 5 and 6.

[0101]
FIG. 8 is a cross section of the structure of the optical waveguide module shown in FIG. 7, cut along the I-I line perpendicular to the optical axis of the optical waveguides 2n (n=1 to 8). In FIG. 8, the portion including the optical waveguides 23 to 25, the light blocking layers 252 to 255, and the photodetectors 633 to 635 is shown enlarged, with the cross section passing through the centers of the photodetectors 633 to 635 (see cross section location B indicated by the dotted line in FIG. 6).

[0102] As shown in FIG. 8, the planar waveguide optical circuit 1 comprises the lower cladding 22 formed on the substrate 10, eight cores 20 corresponding to the optical waveguides 21 to 28 formed in parallel and equidistantly spaced on the lower cladding 22, and the upper cladding 21 formed so as to cover all eight of the cores 20 (the optical waveguides 21 to 28). The photodetector array 62 including the photodetectors 631 to 638 is disposed above the upper cladding 21.

[0103] In this embodiment, the upper cladding 21 is removed from between the optical waveguides 21 to 28 in specific widths of x1, and the light blocking layers 251 to 257 for blocking light are provided in the places where the upper cladding 21 has been removed. As shown in FIG. 7, these light blocking layers 251 to 257 are formed over a specific length 11 spanning the groove 3.

[0104] In the above structure, if we turn our attention to the fourth channel of signal light transmitted through optical waveguide 24, we see that when the signal light transmitted through optical waveguide 24 is incident on the inner filler resin 51 in the groove 3 via the upstream end face 31 (see FIG. 6), the reflection filter 4, which is inclined with respect to the optical axis, reflects part of this signal light diagonally above the planar waveguide optical circuit 1 at a specific reflectivity that has undergone polarization compensation so as to be equal with respect to respective polarization states. The rest of the signal light passes through the inner filler resin 51 and the reflection filter 4, and is again incident on the optical waveguide 24 via the downstream end face 32.

[0105] Meanwhile, the light reflected by the reflection filter 4 is incident on the photodetector 634 via the inner filler resin 51 and the optical waveguide 24. The optical intensity of the fourth channel of signal light transmitted through the optical waveguide 24 is monitored on the basis of the optical intensity of the reflected light detected by the photodetector 634.

[0106] The light blocking layer 253 is provided inside the upper cladding 21 on the side with the optical waveguide 23 and the photodetector 633, as viewed from the optical waveguide 24. Of the signal light transmitted through the optical waveguide 24, any light that propagates to the adjacent photodetector 633 as a result of light scattering or reflection, the spreading of transmitted light, etc., is blocked by this light blocking layer 253, and this prevents crosstalk between the third and fourth channels.

[0107] Also, the light blocking layer 254 is provided inside the upper cladding 21 on the side with the optical waveguide 25 and the photodetector 635, as viewed from the optical waveguide 24. Of the signal light transmitted through the optical waveguide 24, any light that propagates to the adjacent photodetector 635 as a result of light scattering or reflection, the spreading of transmitted light, etc., is blocked by this light blocking layer 254, and this prevents crosstalk between the fourth and fifth channels.

[0108] In this embodiment, light blocking layers 251 to 257 are provided to the reflected light paths from the reflection filter 4 to the photodetectors 631 to 638 as light path isolation means for isolating the reflected light paths from each other within the planar waveguide optical circuit 1. As mentioned above, this effectively prevents the generation of crosstalk between adjacent channels.

[0109] It is preferable for the light blocking layers 251 to 257 provided within the planar waveguide optical circuit 1 to be a light blocking material having the effect of blocking light by absorbing, reflecting, or scattering light of the signal light wavelength, or the like. In specific terms, glass containing a high concentration of an additive having a light absorption effect, such as germanium, can be used, for example, as the light blocking material.

[0110] As shown in FIGS. 7 and 8, the light blocking layers 251 to 257 can be formed, for example, by a method in which seven grooves, each with a width of x1 and a length of l1, are formed substantially parallel to the optical waveguides and spanning the groove 3 between the eight optical waveguides 21 to 28, and the insides of these grooves are filled with a light blocking material. The width x1 and length l1 of the light blocking layers 251 to 257 are preferably set such that the reflected light paths from the reflection filter 4 to the photodetectors 63n are not too narrow, and the light blocking effect can be sufficiently realized between adjacent channels. Alternatively, a light blocking layer can be provided over everything from the input end 11 to the output end 12 of the planar waveguide optical circuit 1.

[0111] As an example, in a state in which crosstalk has worsened to −22 dB, providing a light blocking layer within the upper cladding of the planar waveguide optical circuit as discussed above improves the crosstalk to a level of <−30 dB. Since the conduction of scattered light will be somewhat suppressed even when the inside of the light blocking layer is not filled with anything, an improvement in crosstalk will be obtained in this case as well, with the −22 dB improving to −28 dB, for instance.

[0112]
FIG. 9 is a plan view of the structure in the fourth embodiment of the optical waveguide module. Just as with the first embodiment, this optical waveguide module comprises a planar waveguide optical circuit 1 having a substrate 10 and eight (eight channels) planar waveguide type of optical waveguides 21 to 28 formed on the substrate 10.

[0113] The optical waveguides 21 to 28 are formed parallel to each other and equidistantly, extending from an input end 11 of the planar waveguide optical circuit 1 to an output end 12 in a specific optical transmission direction (the direction indicated by the arrow in FIG. 9). A groove 3 that cuts across the optical waveguides 21 to 28 is provided at a specific location with respect to the optical transmission direction of the planar waveguide optical circuit 1.

[0114] On the inside of this groove 3 is installed a reflection filter 4 for reflecting part of the signal light transmitted through the respective optical waveguides 21 to 28, with the reflection filter 4 being sealed in the groove 3 by a filler resin 5. A sub-mounting substrate 70 is installed above the planar waveguide optical circuit 1 at a location further upstream than the groove 3, and a photodetector array 64 having eight photodetectors 651 to 658 corresponding to the optical waveguides 21 to 28 is installed above the sub-mounting substrate 70 and the filler resin 5.

[0115] In FIG. 9, one-dot chain lines indicate the photodetector array 64 and the photodetectors 651 to 658 in order to illustrate the positional relationship between the photodetector array 64 and the various components, and to show the planar waveguide optical circuit 1, the sub-mounting substrate 70, and so forth in a state in which the photodetector array 64 has been removed. The sub-mounting substrate 70 is a mounting member for mounting the photodetector array 64, and on its top surface are formed wiring, electrodes, etc., for reading out the photodetection signals from the photodetectors 651 to 658, as shown schematically in FIG. 9.

[0116] In this embodiment, a light path isolation means for isolating reflected light paths is provided within the filler resin 5 in between each of the eight reflected light paths along which part of the signal light transmitted through the eight channels of optical waveguides 21 to 28 is reflected by the reflection filter 4 to the corresponding photodetectors 651 to 658. This light path isolation means serves to suppress the generation of crosstalk between adjacent channels.

[0117] In specific terms, in this embodiment the filler resin 5 is installed so as to seal not only the inside of the groove 3, but also a specific portion of the top surface of the planar waveguide optical circuit 1 including the upper part of the groove. Light blocking components 711 to 717 that protrude like the teeth of a comb into the upper filler resin, which seals the top surface of the planar waveguide optical circuit 1, are provided to the sub-mounting substrate 70 located on the upstream side of this upper filler resin.

[0118] Specifically, with respect to the optical waveguides 21 through 28 located within the planar waveguide optical circuit 1, light blocking components 711, 712, 713, 714, 715, 716, and 717 are respectively provided between the optical waveguides 21 and 22, between the optical waveguides 22 and 23, between the optical waveguides 23 and 24, between the optical waveguides 24 and 25, between the optical waveguides 25 and 26, between the optical waveguides 26 and 27, and between the optical waveguides 27 and 28.

[0119] Other than the above-mentioned light blocking components 711 to 717 being provided to the sub-mounting substrate 70, the structure of the optical waveguide module in this fourth embodiment is the same as that in the first embodiment shown in FIGS. 1 and 2.

[0120]
FIG. 10 is a cross section of the structure of the optical waveguide module shown in FIG. 9, cut along the II-II line perpendicular to the optical axis of the optical waveguides 2n (n=1 to 8). In FIG. 10, the portion including the optical waveguides 23 to 25, the light blocking components 712 to 715, and the photodetectors 653 to 655 is shown enlarged, with the cross section passing through the centers of the photodetectors 653 to 655 (see cross section location A indicated by the dotted line in FIG. 2).

[0121] As shown in FIG. 10, the planar waveguide optical circuit 1 comprises the lower cladding 22 formed on the substrate 10, eight cores 20 corresponding to the optical waveguides 21 to 28 formed in parallel and equidistantly spaced on the lower cladding 22, and the upper cladding 21 formed so as to cover all eight of the cores 20 (the optical waveguides 21 to 28).

[0122] The upper filler resin 52, which is the portion of the filler resin 5 that seals a specific portion of the top surface of the planar waveguide optical circuit 1, is provided to the top side of the upper cladding 21 (see FIG. 2). This upper filler resin 52 is substantially the same height as the sub-mounting substrate 70. The photodetector array 64 including the photodetectors 651 to 658 is disposed above the upper filler resin 52 and the sub-mounting substrate 70. The portion under the photodetectors 651 to 658 of the photodetector array 64 is the upper filler resin 52, through which light from the reflection filter 4 is able to pass.

[0123] In this embodiment, the comb-like light blocking components 711 to 717, which are formed at a specific width of x2 at the end faces of the sub-mounting substrate 70 that come into contact with the upper filler resin 52, are provided to the upper filler resin 52 at locations corresponding to between the optical waveguides 21 to 28 (between the photodetectors 651 to 658). The light blocking components 711 to 717 are designed to protrude into the upper filler resin 52, which is downstream of the sub-mounting substrate 70. These light blocking components 711 to 717 are formed over a specific length 12, as shown in FIG. 9.

[0124] In the above structure, if we turn our attention to the fourth channel of signal light transmitted through optical waveguide 24, we see that when the signal light transmitted through optical waveguide 24 is incident on the inner filler resin 51 in the groove 3 via the upstream end face 31 (see FIG. 2), the reflection filter 4, which is inclined with respect to the optical axis, reflects part of this signal light diagonally above the planar waveguide optical circuit 1 at a specific reflectivity that has undergone polarization compensation so as to be equal with respect to respective polarization states. The rest of the signal light passes through the inner filler resin 51 and the reflection filter 4, and is again incident on the optical waveguide 24 via the downstream end face 32.

[0125] Meanwhile, the light reflected by the reflection filter 4 is incident on the photodetector 654 via the inner filler resin 51, the optical waveguide 24, and the upper filler resin 52. The optical intensity of the fourth channel of signal light transmitted through the optical waveguide 24 is monitored on the basis of the optical intensity of the reflected light detected by the photodetector 654.

[0126] The light blocking component 713 is provided inside the upper filler resin 52 on the side with the optical waveguide 23 and the photodetector 653, as viewed from the optical waveguide 24. Of the signal light transmitted through the optical waveguide 24, any light that propagates to the adjacent photodetector 653 as a result of light scattering or reflection, the spreading of transmitted light, etc., is blocked by this light blocking component 713, and this prevents crosstalk between the third and fourth channels.

[0127] Also, the light blocking component 714 is provided inside the upper filler resin 52 on the side with the optical waveguide 25 and the photodetector 655, as viewed from the optical waveguide 24. Of the signal light transmitted through the optical waveguide 24, any light that propagates to the adjacent photodetector 655 as a result of light scattering or reflection, the spreading of transmitted light, etc., is blocked by this light blocking component 714, and this prevents crosstalk between the fourth and fifth channels.

[0128] In this embodiment, light blocking components 711 to 717 are provided to the reflected light paths from the reflection filter 4 to the photodetectors 651 to 658 inside the upper filler resin 52 of the filler resin 5 as light path isolation means for isolating the reflected light paths from each other. As mentioned above, this effectively prevents the generation of crosstalk between adjacent channels.

[0129] It is preferable for the light blocking components 711 to 717 provided to the sub-mounting substrate 70 so as to protrude into the filler resin 5 to be a light blocking material having the effect of blocking light by absorbing, reflecting, or scattering light of the signal light wavelength, or the like. In specific terms, alumina can be used, for example, as the light blocking material.

[0130] When the light blocking components 711 to 717 are formed integrally with the sub-mounting substrate 70 as mentioned above, the sub-mounting substrate 70 itself is formed from a light blocking material, for example. The width x2 and length 12 of the light blocking components 711 to 717 are preferably set such that the reflected light paths from the reflection filter 4 to the photodetectors 65n are not too narrow, and the light blocking effect can be sufficiently realized between adjacent channels.

[0131] As an example, in a state in which crosstalk has worsened to −22 dB, providing light blocking components within the upper filler resin of the filler resin as discussed above improves the crosstalk to a level of <−23 dB.

[0132]
FIG. 11 is a plan view of the structure in the fifth embodiment of the optical waveguide module. Just as with the first embodiment, this optical waveguide module comprises a planar waveguide optical circuit 1 having a substrate 10 and eight (eight channels) planar waveguide type of optical waveguides 21 to 28 formed on the substrate 10.

[0133] The optical waveguides 21 to 28 are formed parallel to each other and equidistantly, extending from an input end 11 of the planar waveguide optical circuit 1 to an output end 12 in a specific optical transmission direction (the direction indicated by the arrow in FIG. 11). A groove 3 that cuts across the optical waveguides 21 to 28 is provided at a specific location with respect to the optical transmission direction of the planar waveguide optical circuit 1.

[0134] On the inside of this groove 3 is installed a reflection filter 4 for reflecting part of the signal light transmitted through the respective optical waveguides 21 to 28, with the reflection filter 4 being sealed in the groove 3 by a filler resin 5. A sub-mounting substrate 72 is installed above the planar waveguide optical circuit 1 at a location further upstream than the groove 3, and a photodetector array 66 having eight photodetectors 671 to 678 corresponding to the optical waveguides 21 to 28 is installed above the sub-mounting substrate 72 and the filler resin 5.

[0135] In FIG. 11, one-dot chain lines indicate the photodetector array 66 and the photodetectors 671 to 678 in order to illustrate the positional relationship between the photodetector array 66 and the various components, and to show the planar waveguide optical circuit 1, the sub-mounting substrate 72, and so forth in a state in which the photodetector array 66 has been removed. The sub-mounting substrate 72 is a mounting member for mounting the photodetector array 66, and on its top surface are formed wiring, electrodes, etc., for reading out the photodetection signals from the photodetectors 671 to 678, as shown schematically in FIG. 11.

[0136] In this embodiment, a light path isolation means for isolating reflected light paths is provided within the planar waveguide optical circuit 1 in between each of the eight reflected light paths along which part of the signal light transmitted through the eight channels of optical waveguides 21 to 28 is reflected by the reflection filter 4 to the corresponding photodetectors 671 to 678. This light path isolation means serves to suppress the generation of crosstalk between adjacent channels.

[0137] In specific terms, in this embodiment a light blocking layer 26 is provided between each of the eight optical waveguides 21 to 28 so as to block light from passing from one reflected light path into an adjacent reflected light path within the planar waveguide optical circuit 1.

[0138] Specifically, light blocking layers 261, 262, 263, 264, 265, 266, and 267 are respectively provided between the optical waveguides 21 and 22, between the optical waveguides 22 and 23, between the optical waveguides 23 and 24, between the optical waveguides 24 and 25, between the optical waveguides 25 and 26, between the optical waveguides 26 and 27, and between the optical waveguides 27 and 28.

[0139] Other than the above-mentioned light blocking layers 261 to 267 being provided to the planar waveguide optical circuit 1, the structure of the optical waveguide module in this fifth embodiment is the same as that in the first embodiment shown in FIGS. 1 and 2.

[0140]
FIG. 12 is a cross section of the structure of the optical waveguide module shown in FIG. 11, cut along the III-III line perpendicular to the optical axis of the optical waveguides 2n (n=1 to 8). In FIG. 12, the portion including the optical waveguides 23 to 25, the light blocking layers 262 to 265, and the photodetectors 673 to 675 is shown enlarged, with the cross section passing through the centers of the photodetectors 673 to 675 (see cross section location A indicated by the dotted line in FIG. 2).

[0141] As shown in FIG. 12, the planar waveguide optical circuit 1 comprises the lower cladding 22 formed on the substrate 10, eight cores 20 corresponding to the optical waveguides 21 to 28 formed in parallel and equidistantly spaced on the lower cladding 22, and the upper cladding 21 formed so as to cover all eight of the cores 20 (the optical waveguides 21 to 28).

[0142] The upper filler resin 52, which is the portion of the filler resin 5 that seals a specific portion of the top surface of the planar waveguide optical circuit 1, is provided to the top side of the upper cladding 21 (see FIG. 2). This upper filler resin 52 is substantially the same height as the sub-mounting substrate 72. The photodetector array 66 including the photodetectors 671 to 678 is disposed above the upper filler resin 52 and the sub-mounting substrate 72. The portion under the photodetectors 671 to 678 of the photodetector array 66 is the upper filler resin 52, through which light from the reflection filter 4 is able to pass.

[0143] In this embodiment, the upper cladding 21 is removed from between the optical waveguides 21 to 28 in specific widths of x3, and the light blocking layers 261 to 267 are provided in the places where the upper cladding 21 has been removed. As shown in FIG. 11, these light blocking layers 261 to 267 are formed over a specific length 13 spanning the groove 3.

[0144] In the above structure, if we turn our attention to the fourth channel of signal light transmitted through optical waveguide 24, we see that when the signal light transmitted through optical waveguide 24 is incident on the inner filler resin 51 in the groove 3 via the upstream end face 31 (see FIG. 2), the reflection filter 4, which is inclined with respect to the optical axis, reflects part of this signal light diagonally above the planar waveguide optical circuit 1 at a specific reflectivity that has undergone polarization compensation so as to be equal with respect to respective polarization states. The rest of the signal light passes through the inner filler resin 51 and the reflection filter 4, and is again incident on the optical waveguide 24 via the downstream end face 32.

[0145] Meanwhile, the light reflected by the reflection filter 4 is incident on the photodetector 674 via the inner filler resin 51, the optical waveguide 24, and the upper filler resin 52. The optical intensity of the fourth channel of signal light transmitted through the optical waveguide 24 is monitored on the basis of the optical intensity of the reflected light detected by the photodetector 674.

[0146] The light blocking layer 263 is provided inside the upper cladding 21 on the side with the optical waveguide 23 and the photodetector 673, as viewed from the optical waveguide 24. Of the signal light transmitted through the optical waveguide 24, any light that propagates to the adjacent photodetector 673 as a result of light scattering or reflection, the spreading of transmitted light, etc., is blocked by this light blocking layer 263, and this prevents crosstalk between the third and fourth channels.

[0147] Also, the light blocking layer 264 is provided inside the upper cladding 21 on the side with the optical waveguide 25 and the photodetector 675, as viewed from the optical waveguide 24. Of the signal light transmitted through the optical waveguide 24, any light that propagates to the adjacent photodetector 675 as a result of light scattering or reflection, the spreading of transmitted light, etc., is blocked by this light blocking layer 264, and this prevents crosstalk between the fourth and fifth channels.

[0148] In this embodiment, just as the light blocking layers 251 to 257 in the third embodiment shown in FIGS. 7 and 8, light blocking layers 261 to 267 are provided to the reflected light paths from the reflection filter 4 to the photodetectors 671 to 678 as light path isolation means for isolating the reflected light paths from each other within the planar waveguide optical circuit 1. As mentioned above, this effectively prevents the generation of crosstalk between adjacent channels.

[0149] With this structure, in which the upper filler resin 52 is provided by filling the space between the photodetector array 66 and the upper cladding 21 of the optical waveguides 21 to 28 with a resin, even if bumps or the like are on the surface of the upper cladding 21, any light scattering or scattered reflection caused by these is suppressed.

[0150] As an example, in a state in which crosstalk has worsened to −20 dB, providing a light blocking layer within the upper cladding of the planar waveguide optical circuit as discussed above improves the crosstalk to a level of <−28 dB.

[0151] The optical waveguide module of the present invention is not limited to the embodiments and examples given above, and a variety of variations is possible. For instance, the polarization compensation of the difference in reflectivity at the reflection filter 4 involves compensating for the difference in reflectivity at the reflection filter 4 itself, but if it is known in advance that polarization dependence will be caused by reflection, etc., at the interfaces of the optical waveguides 2n, the filler resin 5, and the photodetectors 6n, the reflection filter 4 may also be constituted with reflectivity that compensates for these as well.

[0152] Also, the light path isolation means for preventing crosstalk between adjacent channels is not limited to the structure discussed above, and various other structures may be employed instead. For example, when a light blocking member is provided inside the filler resin, it may be a separate member from the sub-mounting substrate, or a light blocking member may be provided inside the inner filler resin. Alternatively, a light blocking member provided inside the upper cladding may be used together with a light blocking member provided inside the filler resin, for instance, thereby further reducing crosstalk.

[0153] As described in detail above, the optical waveguide module according to the present invention offers the following effects. With an optical waveguide module structured for use in monitoring optical intensity, in which an inclined groove is formed cutting across optical waveguides and part of the signal light is reflected by a reflection filter installed inside this groove, the structure and manufacturing process of the optical circuit are simplified. Also, using a reflection filter that has undergone polarization compensation so that the reflectivity is equalized between orthogonal polarization makes it possible to monitor optical intensity accurately regardless of the polarization state of the signal light. Further, since the inside of the groove including the reflection filter is sealed with a filler resin, any deterioration in long-term stability that would otherwise be caused by contamination of these components is prevented.

[0154] An optical waveguide module such as this can be used as a signal light intensity monitor that is inserted into an optical circuit consisting of optical fibers or planar optical waveguides. Alternatively, if this optical waveguide module is provided at the required location in a planar waveguide optical circuit of a optical coupler, optical splitter, optical attenuator, or the like, it will be able to monitor the signal light intensity in the optical circuit.

[0155] Also, crosstalk can be minimized between adjacent channels by providing light path isolation means between N number of adjacent reflected light paths through which N channels of signal light reflected by the reflection filter propagate to photodetectors. This makes it possible to accurately monitor the optical intensity in each channel for N channels of signal light transmitted through the respective optical waveguides.

[0156] From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.

Optical waveguide module

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CPC

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