Patent Publication Number: US-2023161104-A1

Title: Photonic Integrated Optical Device

Description:
TECHNICAL FIELD 
     The present invention relates to an photonic integrated optical device applicable to optical communications systems, and more particularly, to an photonic integrated optical device on which optical circuit elements having optical waveguides as well as optical functional elements such as photodiodes and laser diodes are mounted. 
     BACKGROUND ART 
     Recently with the spread of optical fiber transmission, there has been demand for techniques for integrating a large number of optical circuits at high density. As one of such techniques, a silica-based planar lightwave circuit (hereinafter abbreviated to PLC) is known. The PLC is a waveguide-type optical device with excellent features such as low losses, high reliability, and high design freedom, and a PLC that integrates functions of a multiplexer/demultiplexer, a coupler, and the like is actually mounted on a transmission device at a transmission end of optical communications. 
     Other than the PLC, optical devices mounted in a transmission device include optical functional elements such as photodiodes (hereinafter abbreviated to PDs) configured to convert optical signals into electrical signals and laser diodes (hereinafter abbreviated to LDs) serving as light emission sources. Note that the PD and the LD can be regarded as having an input/output structure of an optical waveguide. In order to further expand channel capacity, there is demand for a highly functional photonic integrated optical device produced by integrating an optical waveguide such as a PLC that performs optical signal processing and optical devices such as PDs that perform photoelectric conversion. 
     As a platform for such an photonic integrated optical device, the PLC is promising, and an optical waveguide component formed by hybrid integration of a PD chip and a PLC chip has been proposed together with a production direction thereof (see Patent Literature 1). Patent Literature 1 describes a method that involves installing a 45-degree mirror in part of a waveguide area, mounting a PD on the waveguide using the mirror to vertically change an optical path of light propagating along the optical waveguide, and optically coupling the PLC to the PD. 
     A device form in which optical functional elements such as a PLC and a PD are mounted in combination in this way is advantageous in terms of downsizing of the device and design freedom of an optical circuit. Also, in order to expand channel capacity, a multichannel integrated device has been developed by optically coupling and mounting a PLC formed by integration of optical circuit functions such as optical signal multiplexing/demultiplexing functions and arrayed multiple optical functional elements. Furthermore, in order to further increase speed and functionality, recently there has been demand for integration of a PLC and an optical functional element having a waveguide structure such as a PD having a waveguide structure suitable for wider bandwidths and an LD having a wavelength tuning function. 
     In such a device form, for example, to butt-join respective input and output waveguides of a PLC and an optical functional element, it is necessary to fix the waveguides to each other. In so doing, because it is difficult to fully eliminate a distance between the waveguides due to processing error and mounting error, optical coupling is actually done such that light will be emitted into space from one of the waveguides and a beam of the light will enter the other waveguide. 
     However, because the emitted beam spreads due to diffraction, an overlap between the emitted beam and a mode field of propagation light on the input-side waveguide is reduced, resulting in a loss, and at the same time reflection occurs due to a difference in index of refraction between end faces of the waveguides and space, resulting in a loss. Consequently, to reduce the losses, it becomes necessary to fill the space with media close in index of refraction to the respective waveguides and transparent to near-infrared light. 
     Thus, a UV- (ultraviolet-) cure adhesive is used commonly, and, for example, in butt-connection of optical fiber and PLC, both the optical fiber and PLC are centered. Subsequently, the ultraviolet-cure adhesive is filled into a gap between an end face of a glass-made fiber block to which the optical fiber is fixed and an end face of the PLC, irradiated with UV light, and consequently able to be cured in a short time. In this way, the end faces can be bonded together more simply and easily than when a thermosetting adhesive is used. If Si-based or InP-based optical functional elements can be integrated by a butt-joining method using a PLC optical circuit as a platform, a more functional photonic integrated optical device can be provided. 
     Actually, however, there are some problems in creating an photonic integrated optical device by integrating optical functional elements by the application of the butt-joining method using a PLC optical circuit as a platform. 
       FIGS.  1 ( a ) and  1 ( b )  are diagrams showing a basic configuration of an example of a well-known photonic integrated optical device  100 A, where  FIG.  1 ( a )  is a plan view of the photonic integrated optical device  100 A as viewed from above and  FIG.  1 ( b )  is a sectional side view of the photonic integrated optical device  100 A taken along line Ib-Ib in  FIG.  1 ( a ) . 
     Referring to  FIGS.  1 ( a ) and  1 ( b ) , the photonic integrated optical device  100 A is configured by butting together a PLC  10  and an optical functional element  20 , filling a UV-cure adhesive into a gap between butted portions, irradiating the adhesive with UV light, and thereby curing the adhesive and forming a joint  30 A. 
     The PLC  10  is configured by laminating an underclad layer  2 , a core layer  3 , and an overclad layer  4 , and thereby forming an optical circuit on top of a first main surface of a substrate  1 . Similarly, the optical functional element  20  is configured by laminating an underclad layer  12 , a core layer  13 , and an overclad layer  14 , and thereby forming an optical circuit on top of a substrate  11 . Here, the PLC  10  is larger in board thickness than the optical functional element  20 . The optical circuit of the PLC  10  is also larger in thickness than the optical circuit of the optical functional element  20 . As a material of the substrates  1  and  11 , Si or InP is used normally. 
     The optical circuit of the PLC  10  is made of silica glass transparent to a wavelength region ranging from the UV band to the near-infrared band while on the other hand, the substrate  1  is made of a material non-transparent to light in a wavelength region ranging from the UV band to the visible light band. These configurations similarly apply to the optical circuit of the optical functional element  20  and to the substrate  11  on the other side of the connection. That is, the substrates  1  and  11 , which are not transparent to light in the wavelength region ranging from the UV band to the visible light band, absorb rays in this wavelength region. 
     To produce the photonic integrated optical device  100 A by butt-joining, the substrates  1  and  11  are created from a material non-transparent to light in the wavelength region ranging from the UV band to the visible light band, combined, and joined together. In this case, the core layers  3  and  13 , which are to become the optical waveguides of the optical circuits on top of the substrates  1  and  11 , are centered with respect to each other, and then a UV-cure adhesive is filled into a gap between end faces of these parts. Then, when UV light is emitted vertically from above, the optical circuits as well as the substrates  1  and  11  are fixed together adhesively, allowing the joint  30 A to be formed. 
     However, the emitted UV light, which is absorbed by the substrates  1  and  11  without being transmitted therethrough, is unreachable to most of a region to be bonded as with a region E1 shown in  FIG.  1 ( b ) , leaving the region uncured. As a result, the UV-cure adhesive cannot be cured completely, and in an incompletely cured state, there is a problem in that sufficient bonding strength is not available in forming the joint  30 A in terms of the structure of the photonic integrated optical device  100 A. 
       FIGS.  2 ( a ) and  2 ( b )  are diagrams showing a basic configuration of another example of a well-known photonic integrated optical device  100 B, where  FIG.  2 ( a )  is a plan view of the photonic integrated optical device  100 B as viewed from above and  FIG.  2 ( b )  is a sectional side view of the photonic integrated optical device  100 B taken along line IIb-IIb in  FIG.  2 ( a ) . 
     Referring to  FIGS.  2 ( a ) and  2 ( b ) , in the photonic integrated optical device  100 B, when the PLC  10  and the optical functional element  20  are butted together, butt-joint holding substrates  15  and  16  equal in board thickness are provided on the side of the optical circuits on top of the substrates  1   and  11 . It is assumed that a glass material that transmits UV light is used for the substrates  15  and  16 , for example. Then, a UV-cure adhesive is filled into a gap between butted portions, irradiated with UV light, and thereby cured to form a joint  30 B. 
     Thus, to produce the photonic integrated optical device  100 B by butt-joining, the substrates  15  and  16  are mounted on top of the optical circuits on the PLC  10  and optical functional element  20 , combined, and joined together. In this case, the core layers  3  and  13 , which are to become the optical waveguides of the optical circuits are centered with respect to each other, and then, when the PLC  10  and the optical functional element  20  are butt-joined, the substrates  15  and  16  on top thereof are butted together at the same time, and a UV-cure adhesive is filled into a gap between end faces of these parts. Then, when UV light is emitted obliquely from above, the substrates  1  and  11  are fixed together adhesively, so are the optical circuits, as well as the substrates  15  and  16 , allowing the joint  30 B to be formed. 
     However, it is often necessary to provide free space on the optical circuit on top of the optical functional element  20  for the purpose of installing an electrode pad for use to drive, for example, a LD, a PD, or the like. In such a case, in the optical circuit of the optical functional element  20 , a region E2 shown in  FIG.  2 ( b )  becomes an extra occupation area to mount the substrate  16 , making it difficult to mount the substrate  16  while providing necessary free space. In other words, the occupation area used to mount the substrate  16  acts as an impediment to securing necessary free space. To provide a special area separately to mount such a substrate  16  is not a good measure because such a measure will increase the size of the optical functional element  20  and decrease the level of integration. 
       FIGS.  3 ( a ) and  3 ( b )  are diagrams showing a basic configuration of still another example of a well-known photonic integrated optical device  100 C, where  FIG.  3 ( a )  is a plan view of the photonic integrated optical device  100 C as viewed from above and  FIG.  3 ( b )  is a sectional side view of the photonic integrated optical device  100 C taken along line IIIb-IIIb in  FIG.  3 ( a ) . 
     Referring to  FIGS.  3 ( a ) and  3 ( b ) , in the photonic integrated optical device  100 C, when the PLC  10  and the optical functional element  20  are butted together, butt-joint holding substrate  17  and  18  equal in board thickness are provided on undersides of second main surfaces of the respective substrates  1  and  11 . However, it is assumed that a glass material that transmits, for example, UV light is used for the substrates  17  and  18 . Then, a UV-cure adhesive is filled into a gap between butted portions, irradiated with UV light, and thereby cured to form a joint  30 C. 
     Thus, to produce the photonic integrated optical device  100 C by butt-joining, other substrates  17  and  18  are mounted on undersides of the substrates  1  and  11  of the PLC  10  and the optical functional element  20 , combined, and joined together. In this case, the core layers  3  and  13 , which are to become the optical waveguides of the optical circuits are centered with respect to each other, and then, when the PLC  10  and the optical functional element  20  are butt-joined, the substrates  17  and  18  on undersides of the PLC  10  and the optical functional element  20  are butted together at the same time, and a UV-cure adhesive is filled into a gap between end faces of these parts. Then, when UV light is emitted obliquely from below, the substrates  17  and  18  are fixed together adhesively, so are the optical circuits, as well as the substrates  1  and  11 , allowing the joint  30 C to be formed. 
     However, when other substrates  17  and  18  are mounted on the undersides of the substrates  1  and  11 , although the substrates  17  and  18  can be fixed adhesively to each other by avoiding the problem with the photonic integrated optical device  100 B, a large amount of the UV-cure adhesive remains uncured on end faces of the substrates  1  and  11 . A reason for this is that UV light is absorbed by the substrates  1  and  11  without being transmitted therethrough as described above and the UV-cure adhesive in a region E3 shown in  FIG.  3 ( b )  is left uncured. As a result, as with the photonic integrated optical device  100 A, the UV-cure adhesive cannot be cured completely, and in an incompletely cured state, there is a problem in that sufficient bonding strength is not available in forming the joint  30 C in terms of the structure of the photonic integrated optical device  100 C. 
     That is, with the photonic integrated optical devices  100 A and  100 C, the formation of the joints  30 A and  30 C, which do not provide sufficient bonding strength, cannot be said to be an effective technique from the viewpoint of long-term reliability. With the photonic integrated optical device  100 B, the joint holding substrate  16  is not easy to install on the optical circuit on top of the substrate  11  of the optical functional element  20  and acts as an impediment to securing free space necessary for the optical circuit, and thus, this cannot be said to be an effective technique. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Laid-Open No. 2005-70365 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     The present invention has been made to solve the above problems. An object of an embodiment of the present invention is to provide an photonic integrated optical device that allows hybrid integration of an optical functional element to be implemented simply and easily using a PLC optical circuit as a platform and allows high accuracy butt-joining of optical waveguides. 
     Means for Solving the Problem 
     To achieve the above object, according to one aspect of the present invention, there is provided an photonic integrated optical device, comprising: a planar lightwave circuit made up of an optical circuit made of a material transparent to light in a wavelength region ranging from an ultraviolet band to a visible light band provided on top of a first main surface of a substrate made of a material non-transparent to light in the wavelength region ranging from the ultraviolet band to the visible light band; an optical functional element made up of an optical circuit made of the material transparent to light in the wavelength region ranging from the ultraviolet band to the visible light band provided on an underside of a second main surface of a substrate made of the material non-transparent to light in the wavelength region ranging from the ultraviolet band to the visible light band; a butt-joint made of the material transparent to light in the wavelength region ranging from the ultraviolet band to the visible light band, and holding substrate installed on top of the optical circuit of the planar lightwave circuit and used for joining by means of a ultraviolet-cure adhesive by being butted against an end face of the optical functional element; and, a joint in which respective optical waveguides of the optical circuits are centered with respect to each other, the joint including the ultraviolet-cure adhesive used to butt-join the planar lightwave circuit and the optical functional element, wherein the joint is formed with the ultraviolet-cure adhesive being cured after being filled into a gap between the optical circuit of the optical functional element and the optical circuit of the planar lightwave circuit and a gap between an end face of the substrate of the optical functional element and an end face of the butt-joint holding substrate. 
     Effects of the Invention 
     The structure according to the aspect described above makes it possible to butt together the optical waveguides accurately and thereby obtain sufficient bonding strength when forming the joint by curing the UV-cure adhesive after the respective optical waveguides of the optical circuits are centered with respect to each other by butting together the PLC and the optical functional element. This makes it possible to provide an photonic integrated optical device that allows hybrid integration of an optical functional element to be implemented simply and easily using a PLC optical circuit as a platform and allows high accuracy butt-joining of optical waveguides, at low cost. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS.  1 ( a ) and  1 ( b )  are diagrams showing a basic configuration of an example of a well-known photonic integrated optical device, where  FIG.  1 ( a )  is a plan view of the photonic integrated optical device as viewed from above and  FIG.  1 ( b )  is a sectional side view of the photonic integrated optical device taken along line Ib-Ib in  FIG.  1 ( a ) . 
         FIGS.  2 ( a ) and  2 ( b )  are diagrams showing a basic configuration of another example of a well-known photonic integrated optical device, where  FIG.  2 ( a )  is a plan view of the photonic integrated optical device as viewed from above and  FIG.  2 ( b )  is a sectional side view of the photonic integrated optical device taken along line IIb-IIb in  FIG.  2 ( a ) . 
         FIGS.  3 ( a ) and  3 ( b )  are diagrams showing a basic configuration of still another example of a well-known photonic integrated optical device, where  FIG.  3 ( a )  is a plan view of the photonic integrated optical device as viewed from above and  FIG.  3 ( b )  is a sectional side view of the photonic integrated optical device taken along line IIIb-IIIb in  FIG.  3 ( a ) . 
         FIGS.  4 ( a ) and  4 ( b )  are diagrams showing a basic configuration of an photonic integrated optical device according to Embodiment 1 of the present invention, where  FIG.  4 ( a )  is a plan view of the photonic integrated optical device as viewed from above and  FIG.  4 ( b )  is a sectional side view of the photonic integrated optical device taken along line IVb-IVb in  FIG.  4 ( a ) . 
         FIG.  5    is a perspective view showing a basic configuration of an photonic integrated optical device according to Embodiment 2 of the present invention as viewed obliquely from above. 
         FIGS.  6 ( a ) to  6 ( c ) are enlarged views showing a butt-joined portion of the photonic integrated optical device shown in  FIG.  5   , where  FIG.  6 ( a )  is a top view of the butt-joined portion,  FIG.  6 ( b )  is a sectional side view taken along line VIb-VIb in  FIG.  6 ( a ) , and  FIG.  6 ( c )  is a plan view of the butt-joined portion during prealignment with a marker used in a place where propagation of light through core layers is not obstructed. 
         FIG.  7    is a diagram showing evaluation results on bonding strength as measured by optical coupling loss in the photonic integrated optical device shown in  FIG.  5   , where the loss is measured by applying a force to a PD in a direction horizontal to butt-joining end faces after light sensitivity of the PD is measured on a channel by channel basis. 
         FIG.  8    is a perspective view showing a basic configuration of an photonic integrated optical device according to Embodiment 3 of the present invention as viewed obliquely from above. 
         FIGS.  9 ( a ) to  9 ( d )  are enlarged views showing a butt-joined portion of the photonic integrated optical device shown in  FIG.  8   , where  FIG.  9 ( a )  is a top view of the butt-joined portion,  FIG.  9 ( b ) is a sectional side view taken along line IXb-IXb in  FIG.  9 ( a ) ,  FIG.  9 ( c )  is a plan view of the butt-joined portion during alignment using a groove and a groove marker, and  FIG.  9 ( d )  is a plan view of the butt-joined portion during prealignment using metal markers. 
         FIG.  10    is a diagram showing evaluation results on bonding strength as measured by optical coupling loss in the photonic integrated optical device shown in  FIG.  8   , where the loss is measured by applying a force to a PD in a direction horizontal to butt-joining end faces after light sensitivity of the PD is measured on a channel by channel basis. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Photonic integrated optical devices according to a few embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
     Embodiment 1 
       FIGS.  4 ( a ) and  4 ( b )  are diagrams showing a basic configuration of an photonic integrated optical device  100 D according to Embodiment 1 of the present invention, where  FIG.  4 ( a ) is a plan view of the photonic integrated optical device  100 D as viewed from above and  FIG.  4 ( b )  is a sectional side view of the photonic integrated optical device  100 D taken along line IVb-IVb in  FIG.  4 ( a ) . 
     Referring to  FIGS.  4 ( a ) and  4 ( b ) , in the photonic integrated optical device  100 D, a PLC  10  and an optical functional element  20  are butt-joined together using a butt-joint holding substrate  19  mounted and fixed to top of a first main surface of a substrate  1  on the side of an optical circuit. However, it is assumed that a glass material that transmits, for example, light in a wavelength region ranging from the UV band to the visible light band is used for the substrate  19 . Regarding the optical functional element  20 , the substrate  11  is used by being turned over such that a second main surface of the substrate  11  will be on the top side, and a UV-cure adhesive is filled into a gap between butted portions, irradiated with UV light, and thereby cured, forming a joint  30 D. 
     Again, the PLC  10  is configured by laminating an underclad layer  2 , a core layer  3 , and an overclad layer  4 , thereby forming an optical circuit on top of a first main surface of the substrate  1 . Similarly, the optical functional element  20  is configured by laminating an underclad layer  12 , a core layer  13 , and an overclad layer  14 , thereby forming an optical circuit on top of the substrate  11 . Here, the PLC  10  is slightly larger in board thickness than the optical functional element  20 . The optical circuit of the PLC  10  is also larger in thickness than the optical circuit of the optical functional element  20 . As a material of the substrates  1  and  11 , Si or InP is used normally. 
     That is, the photonic integrated optical device  100 D, includes the butt-joint holding substrate  19  installed on top of the optical circuit of the PLC  10 , and used for joining by means of an UV-cure adhesive by being butted against an end face of the optical functional element  20 . The substrate  1  of the PLC  10  and the substrate  11  of the optical functional element  20  are made of a material non-transparent to light in a wavelength region ranging from a UV band to a visible light band. In contrast, the optical circuit of the PLC  10 , the optical circuit of the optical functional element  20 , and the substrate  19  are made of a transparent material that transmits light in the wavelength region ranging from the UV band to the visible light band. The joint  30 D described above is formed by a UV-cure adhesive cured after being filled into a gap between the optical circuit of the optical functional element  20  and the optical circuit of the PLC  10 . The joint  30 D is formed by a similar UV-cure adhesive cured after being filled into a gap between an end face of the substrate  11  of the optical functional element  20  and an end face of the butt-joint holding substrate  19 . 
     That is, in the PLC  10  of the photonic integrated optical device  100 D, the substrate  19  is mounted on top of the substrate  1  using an adhesive or the like such that an end face of the optical circuit on top of the substrate  1  and the end face of the butt-joint holding substrate  19  will be flush with an output end face of the optical circuit. Also, in the optical functional element  20 , the core layer  13  in which the optical waveguide of the optical circuit on an underside of the substrate  11  is formed and the core layer  3  in which the optical waveguide of the optical circuit on top of the substrate  1  of the PLC  10  is formed are butted together such that layer surfaces of the core layers  3  and  13  will be parallel to each other. In this way, the end faces of the respective optical circuits of the PLC  10  and optical functional element  20  are butted together and centered. In so doing, as the substrate  1  of the PLC  10  and the substrate  11  of the optical functional element  20  are placed in a diagonal direction, the end face of the butt-joint holding substrate  19  and the end face of the optical circuit of the PLC  10  are butted against the end face of the optical circuit of the optical functional element  20  and the end face of the substrate  11 . 
     In the photonic integrated optical device  100 D, it is assumed that with the PLC  10  and the optical functional element  20  being butted together, the optical waveguides of the respective optical circuits are centered with respect to each other, and then the UV-cure adhesive is cured, thereby forming the joint  30 D. The photonic integrated optical device  100 D is structured such that in forming the joint  30 D, the optical waveguides will be butted against each other accurately to provide sufficient bonding strength. Consequently, hybrid integration of the optical functional element  20  can be implemented simply and easily using the optical circuit of the PLC  10  as a platform and the optical waveguides can be butt-joined accurately, and thus the photonic integrated optical device  100 D can be provided at low cost. 
     Giving a concrete description with reference to  FIGS.  4 ( a ) and  4 ( b ) , in the PLC  10 , with reference to a top side of the substrate  1  made of Si that does not transmit light ranging from UV to visible light, the optical functional element  20  and the core layer  3  for input and output of signals are provided above a neighborhood of an end face of the substrate  1 . The butt-joint holding substrate  19  is mounted and fixed to top of the substrate  1  using an adhesive or the like such that the end face of the substrate  19  that transmits light in a region ranging from the UV band to the visible light band will be flush with an input/output end face of an optical circuit of the substrate  1 . This configuration may be called an optical circuit chip. 
     In contrast, in the optical functional element  20 , an optical circuit is provided on an underside of the substrate  11  made of Si or InP that does not transmit light ranging from UV to visible light and the core layer  13  for input and output of signals are provided near the end face of the optical circuit with reference to an underside of the substrate  11 . This configuration may be called an optical functional element chip. The cores of the optical waveguides are centered with respect to each other via the end faces of the optical circuit chip and optical functional element chip such that the core layer  3  of the optical circuit chip and the core layer  13  of the optical functional element chip will have layer surfaces parallel to each other. At the same time, the substrate  1  of the optical circuit chip and the substrate  11  of the optical functional element chip are placed in a diagonal direction with respect to the layer surfaces of the core layers  3  and  13 . Consequently, the end face of the butt-joint holding substrate  19  and the end face of the substrate  11  of the optical functional element chip are placed facing each other on butt-joining end faces. 
     Thus, as shown in  FIG.  4 ( b ) , the joint  30 D can be formed by curing the UV-cure adhesive filled into a gap between the butt-joining end faces with UV light transmitted through the butt-joint holding substrate  19 . This makes it possible to increase bonding area and enable butt-joining of the two chips with increased bonding strength, without providing a fixing substrate on the side of the optical functional element  20 . 
     In the photonic integrated optical device  100 D described above, the butt-joint holding substrate  19  that transmits light in the region ranging from the UV band to the visible light is provided on top of the PLC  10 , and the substrates  1  and  11  are placed in a diagonal direction with respect to the layer surfaces of the core layers  3  and  13  during butt-joining. This configuration makes it possible to increase bonding area and thereby increase bonding strength without providing a fixing substrate on the optical functional element  20 , and thus can reduce the number of parts and man-hours, eliminating the need to change the design of the optical functional element  20 . Besides, being able to greatly reduce the amount of adhesive left uncured without transmitting UV light, the configuration is desirable from the viewpoint of long-term reliability. This makes it possible to implement butt-joining of the optical circuit chip and the optical functional element chip simply and easily. 
     Generally, in a sectional structure of the PLC  10 , as an example, a SiO 2  thin film is deposited to a thickness of approximately 20 µm as an underclad layer  2 , a SiO 2  thin film doped with Ge and the like and higher in index of refraction than the clad layers is deposited to a thickness of 3 to 10 µm as a core layer  3 , and a SiO 2  thin film is deposited to a thickness of approximately 20 µm as an overclad layer  4 , on top of a Si substrate  1 . In this way, a basic structure of the PLC  10  in which the optical circuit of the PLC  10  is provided on the Si substrate  1  is applied to Embodiment 1. 
     In a sectional structure of the optical functional element  20 , as an example, a SiO 2  thin film is deposited to a thickness of a few µm as an underclad layer  12 , a Si thin film is deposited to a thickness of a few hundred nm as a core layer  13 , and a SiO 2  thin film is deposited to a thickness of a few µm as an overclad layer  14 , on the underside of the Si substrate  11 . In this way, a basic configuration of the optical functional element  20  in which a Si photonic optical circuit is provided on the underside of the Si substrate  11  is applied to Embodiment 1. 
     Note that in the optical functional element  20  that uses an InP substrate  11 , as an example, assuming that the substrate  11  is assimilated with the underclad layer  12 , a compound semiconductor is deposited to a thickness of a few hundred nm as a core layer  13  and an InP film or a SiN or SiO 2  thin film for passivation is deposited as an overclad layer  14 . By assuming the core layer  13  formed in an end face area of the substrate  11  to be an optical waveguide for input and output of signals, optical coupling is done in a mode field on an end face. 
     A UV-cure adhesive is filled into a gap between the optical circuit of the optical functional element  20  and the optical circuit of the PLC  10  and a gap between an end face of the substrate  11  of the optical functional element  20  and an end face of the butt-joint holding substrate  19 . Consequently, the joint  30 D is formed with the UV-cure adhesive being cured. In so doing, to increase the strength of adhesive fixing to the optical functional element  20 , desirably the substrate  19  is equal to or larger than the substrate  11  in thickness. This will cause UV light to reach the UV-cure adhesive sufficiently and allows filling condition of the UV-cure adhesive to be checked. As a transparent material that transmits light in a wavelength region ranging from the UV band to the visible light band, desirably silica glass, which is a vitreous material, is used for the substrate  19 . 
     On the end face of the substrate  19 , preferably the thickness of the UV-cure adhesive is controlled according to optical coupling efficiency and adhesive curing conditions, making it possible to adjust the thickness of the UV-cure adhesive filled into a gap between an end face of the optical circuit on top of the substrate  1  and an end face of the optical functional element  20 . In so doing, desirably a bonding end face of the optical circuit on top of the substrate  1  and a bonding end face of the optical circuit on the underside of the substrate  11  are installed so as to be flush with each other. 
     In performing centering of respective optical waveguides (cores) of the core layers  3  and  13  with respect to each other during butt-joining, in order to perform alignment efficiently, rough alignment is often performed through image observation as a pre-stage. According to the present embodiment, the substrates  1  and  11  are placed in a diagonal direction with respect to the layer surfaces of the core layers  3  and  13 . For that, in the alignment of the core layers  3  and  13 , in order to observe the optical circuits or the substrates  1  and  11  not only from above, but also from below, it is necessary to install an additional device such as a camera. In such a case, however, in joining together a particularly small optical functional element  20  and PLC  10 , there is a problem in that the camera on the underside will cause interference. 
     Thus, desirably alignment markers for use during butting are provided near an input/output end face of the optical circuit of the PLC  10  as well as near an input/output end face of the optical circuit of the optical functional element  20 . The alignment markers can be provided in such places on butt-joining end faces between the optical circuit of the PLC  10  and the optical circuit of the optical functional element  20  that do not obstruct light propagation through the core layers  3  and  13  on which optical waveguides are formed. Then, the light can be observed through the substrate  19  in a direction diagonal to a direction normal to the plane of the substrate  1  of the optical circuit of the PLC  10 . This makes it possible to easily implement alignment without the need to add a camera. The core layers  3  and  13 , whose widths are on the order of a few microns, are difficult to align accurately through camera observation, but the core layers  3  and  13  can be observed easily even through a camera if a marker larger in size than the core layers  3  and  13  is provided separately near the butt-joining end faces. 
     Furthermore, the use of a vernier marker will make it possible to implement alignment with submicron level high accuracy even in the case of camera observation. Such a marker structure, if provided in such a place that do not obstruct light propagation by avoiding the optical waveguides (cores) of the core layers  3  and  13 , can be introduced into the PLC  10  and the optical functional element  20  without requiring any additional process. Specifically, possible places include a SiO 2  layer containing dopants in the case of the PLC  10 , a Si layer in the case of Si photonics on an optical circuit, and a compound semiconductor in the case of InP. In so doing, if camera observation is carried out in a vertical direction of the butt-joining end faces, the core layers  3  and  13 , which are low in height due to Si photonics or InP, are difficult to observe. Thus, if camera observation is carried out diagonally with respect to a direction normal to the plane of the substrate  11  of the optical functional element  20  as with the present embodiment, the top faces of the core layers  3  and  13  rather than end faces of the core layers  3  and  13  can be observed obliquely. The observation carried out in this way improves visibility of the core layers  3  and  13  because the core layers  3  and  13  are stretched in a horizontal direction of the substrates  1  and  11  and the depth is observed as being extended by just that much. 
     Therefore, desirably a marker is provided near the end faces where light propagation through the core layers  3  and  13  is not obstructed. In the PLC  10 , since the SiO 2  core layer  3  containing dopants exists between the SiO 2  underclad layer  2  and overclad layer  4 , the core layer  3  has a relatively small difference in index of refraction from the underclad layer  2  and the overclad layer  4 . This results in low contrast, which makes the contour of a structure look blurred. In the case of the optical functional element  20 , when the core layer  13  is observed obliquely in the horizontal depth direction of the substrate  11 , the length in the depth direction is limited by the thickness of the underclad layer  12 . In such a case, in order to perform alignment more accurately, desirably a marker is provided near the butt-joining end faces by creating a metal pattern on the top face of the overclad layer  14 . 
     The use of a metal marker on the top face of the PLC  10  improves visibility compared to the marker on the core layer  3 . On the optical functional element  20 , since the thickness of the metal pattern on the top face of the overclad layer  14  is added to the thickness of the underclad layer  12 , the length of the marker in the horizontal depth direction of the substrate  11  is increased in observation range accordingly when the marker is observed obliquely. In the case of an array joint that includes two or more combinations of the core layers  3  and  13  to be butt-joined, in order to perform alignment accurately, desirably corresponding markers are provided at two or more locations on respective end faces of the optical circuit and the optical functional element  20 . If alignment is performed including the butt-joining end faces and vertical axis such that the markers will be aligned with each other at two locations, rotations around an axis vertical to the butt-joining end faces can also be aligned through camera observation. 
     The marker-based alignment described above enables accurate alignment in the horizontal direction of the substrates  1  and  11 . In contrast, in order to accurately perform alignment in the directions normal to the planes of the substrates  1  and  11 , desirably a groove deeper than the underclad layer  12  is provided in the joining end face of the optical functional element  20 . For example, if the depth of the groove is set equal to the thickness of the overclad layer  14  on the side of the optical circuit plus half the thickness of the core layer  13 , a bottom face of the groove in the end face of the optical functional element  20  corresponds to the overclad layer  14  of the optical circuit, in a butt-joining structure. This makes it possible to improve visibility in camera observation and perform alignment in the direction normal to the substrate  11  accurately. Furthermore, in filling the UV-cure adhesive into a gap between the butt-joining end faces, the existence of the groove in the end face of the optical functional element  20  makes it possible to keep the adhesive from flowing out to a surface of the optical functional element  20  and thereby achieve stable adhesive fixing. 
     In this way, the optical circuits provided on top of the substrates  1  and  11 , respectively, and transparent to light in the region ranging from the UV band to the visible light are butt-joined using the substrate  19  provided on top of the PLC  10  and transparent to light in the region ranging from the UV band to the visible light, to achieve hybrid integration. In this configuration of the photonic integrated optical device, the joint  30 D is formed by filling the UV-cure adhesive into a gap between the end faces of the optical circuits toward the top faces of the substrates  1  and  11  such that the end face of the butt-joint holding substrate  19  will be flush with the end faces of the core layers  3  and  13  of the optical circuits, and thereby form the joint  30 D. The layer surfaces of the core layers  3  and  13  of the respective optical circuits are centered via the end faces of the optical circuit chip and the optical functional element chip so as to be parallel to each other. If the substrate  1  and the substrate  11  are placed diagonally with respect to the layer surfaces of the core layers  3  and  13 , the end face of the substrate  19  and the end face of the substrate  11  are placed facing each other as butt-joining end faces. This makes it possible to cure the UV-cure adhesive filled into the gap between the butt-joining end faces, using UV light passing through the substrate  19  and thereby form the joint  30 D. This in turn makes it possible to improve bonding strength without providing any substrate for additional fixing or any mounting area on the side of the optical functional element  20  and thereby obtain the photonic integrated optical device  100 D butt-joined simply and easily with high accuracy. 
     Embodiment 2 
       FIG.  5    is a perspective view showing a basic configuration of an photonic integrated optical device  100 E according to Embodiment 2 of the present invention as viewed obliquely from above. 
     Referring to  FIG.  5   , the photonic integrated optical device  100 E differs from the photonic integrated optical device  100 D in that a PD  20 A is used for the optical functional element  20 . Regarding the PLC  10 , whereas a light input unit 10 IN  and light output unit 10 OUT  for the core layer  3  are shown here, the substrate  19  is provided on top of the optical circuit here again. Regarding the PD  20 A, an input optical waveguide unit 20A IN  and a Ge-based photoelectric conversion unit  20 A a  are shown. Again, in the photonic integrated optical device  100 E, the UV-cure adhesive is filled into a gap between the optical circuit of the PD  20 A and the optical circuit of the PLC  10  and a gap between the end face of the substrate  11  of the PD  20 A and the end face of the substrate  19 . Consequently, the joint  30 D is formed with the UV-cure adhesive being cured. 
     A Si substrate  1  measuring 5 mm in length, 10 mm in width, and 1 mm in board thickness was used for the silica-based PLC  10  making up the photonic integrated optical device  100 E. A SiO 2  underclad layer  2  with a layer thickness of 20 µm, a SiO 2  core layer  3  with a core width of 4.5 µm and a film thickness of 4.5 µm, and a SiO 2  overclad layer  4  with a layer thickness of 15.5 µm were laminated on top of the substrate  1 . The optical circuit configured in this way may be called a PLC chip. Here, an optical waveguide was formed by the core layer  3  such that the core of the core layer  3  will differ by 2.0% in index of refraction from the overclad layer  4  and the underclad layer  2 . The PLC  10  receives input of light through the light input unit 10 IN  provided on a short side by being formed by the core layer  3  serving as the optical waveguide, and outputs the light through the light output unit 10 OUT  formed on a short side on the opposite side when viewed from the light input unit 10 IN  after the light has propagated through the core layer  3 . That is, four channels of the optical waveguide are installed with a pitch of 250 µm here and are each provided with an S-shaped structure running in a route from the light input unit 10 IN  to the photoelectric conversion unit  20 A a  of the PD  20 A. 
     A Si substrate  11  measuring 1.5 mm in length, 1.5 mm in width, and 0.625 mm in board thickness was used for the PD  20 A to be butt-joined to the PLC  10 . Then, a SiO 2  underclad layer  12  with a film thickness of 3.0 µm, a Si core layer  3  with a core width of 0.5 µm and a film thickness of 0.22 µm, and a SiO 2  overclad layer  14  with a film thickness of 1.5 µm were laminated on the underside of the substrate  11 . The optical circuit configured in this way may be called a Si photonics chip. Here, a spot size converter is used for the input optical waveguide unit 20A IN  and a mode field diameter (MFD: full width which makes intensity equal to 1/e 2  in a light intensity distribution) of the spot size converter is 3 µm both in vertical and horizontal directions of the PD  20 A. The light inputted through the input optical waveguide unit 20A IN  propagates through the core layer  13  and is converted into an electrical signal by the photoelectric conversion unit  20 A a . Light sensitivity of each light-receiving unit of the PD  20 A excluding optical coupling loss is 1.0A/W at a wavelength of 1.55 µm. 
     The butt-joint holding substrate  19  measuring 3 mm in length, 2 mm in width, and 0.7 mm in board thickness is made of synthetic silica, which is a glass material, and is fixed to a top face of the PLC  10  with an adhesive such that an end face will be flush with a light output end face of the PLC  10 . By making the substrate  19  larger in board thickness than the substrate  11  of the PD  20 A, it is possible to bond the entire end face of the PD  20 A and thereby improve bonding strength. 
     To butt-join the PD  20 A to the PLC  10  equipped with the substrate  19 , the substrate  1  of the PLC  10  and the substrate  11  of the PD  20 A are placed in a diagonal direction with respect to the layer surfaces of the respective core layers  3  and  13  of the PLC  10  and PD  20 A. Subsequently, the butt-joining end faces of the PLC  10  and the PD  20 A are prealigned by being observed from above and below. Furthermore, to maximize the light sensitivity of the PD  20 A to the light outputted from the light output unit 10 OUT  of the PLC  10 , precise alignment is performed such that the position of the output light from the core layer  3  of the PLC  10  will accurately match the position of the input light to the core layer  13  of the PD  20 A. In so doing, alignment can be performed effectively if observation is carried out obliquely upward through the substrate  19 . 
     Then, to fix the butt-joint between the PD  20 A and the PLC  10 , the gap between the PLC  10  and the PD  20 A is filled with a UV-cure adhesive transparent to the infrared region and close in index of refraction to the core layer  3 , overclad layer  4 , and underclad layer  2  of the PLC  10 . Subsequently, as shown in  FIG.  4 ( b ) , UV light is irradiated obliquely from above via the substrate  19  to cure the adhesive, thereby forming the joint 3D. If a transparent substrate  19  that transmits light in the region ranging from the UV band to the visible light is used, the entire butt-joining end face of the PD  20 A can be irradiated by UV light, making it possible to cure the UV-cure adhesive sufficiently and directly check filling condition of the UV-cure adhesive and connection state of the butt-joining end face of the PD  20 A. To enable making such direct checks by means of a camera or the like, desirably a surface of the substrate  19  serving as an observation path is a polished surface so as to allow observation of the butt-joining end face as well as to be transparent to light in the region ranging from the UV band to the visible light. In so doing, desirably an antireflection film having an index of refraction corresponding to the index of refraction of the UV-cure adhesive to be filled is provided on an input/output end face of the PD  20 A. 
     Whereas the prealignment described above is normally performed from above and below through camera observation, a simpler and easier technique is applied in Embodiment 2.  FIGS.  6 ( a ) to  6 ( c )  are enlarged views showing a butt-joined portion of the photonic integrated optical device  100 E, where  FIG.  6 ( a )  is a top view of the butt-joined portion,  FIG.  6 ( b )  is a sectional side view taken along line VIb-VIb in  FIG.  6 ( a ) , and  FIG.  6 ( c )  is a plan view of the butt-joined portion during prealignment with a marker used in a place where propagation of light through core layers  3  and  13  is not obstructed. 
     Specifically, as shown in  FIG.  6 ( a ) , Au markers M for use during alignment are provided at least near an input/output end face of the core layer  3  of the PLC  10  and near an input/output end face of the core layer  13  of the PD  20 A. Then, as shown in  FIG.  6 ( b ) , if observation is carried out obliquely from above through the substrate  19  and adjustments are made such that patterns of the markers M will align with each other, prealignment can be performed without the need to prepare a camera for observation from below. If a butt-joining end face is observed obliquely from above, a pattern in a depth direction of the optical waveguide in the core layer  13  can be observed transparently through the underclad layer  12  of the PD  20 A as shown in  FIG.  6 ( c ) . If a vernier pattern is used as a pattern of the markers M and the scale width and vernier division are set to 10 µm and 0.5 µm, respectively, while the optical waveguide (core) in the core layer  13  of the PD  20 A is 0.5 µm wide (which may be smaller) the markers M are reduced to such a size as to be easy to observe even through a camera. Consequently, along with vernier-based alignment, prealignment can be performed with high accuracy. 
     If the markers M are provided in two or more places near the end faces of the butt-joint and alignment adjustments are made such that patterns of the markers M will align with each other, rotation runout around an axis vertical to the butt-joining end faces can also be corrected. In this way, if markers M for alignment are provided near the butt-joining end faces of the PLC  10  and PD  20 A, this is effective in improving alignment accuracy. The photonic integrated optical device  100 E was produced in this way. 
       FIG.  7    is a diagram showing evaluation results on bonding strength as measured by optical coupling loss [dB] in the photonic integrated optical device  100 E, where the loss is measured by applying a force to a PD  20 A in a direction horizontal to butt-joining end faces after light sensitivity of the PD  20 A is measured on a channel by channel basis. However,  FIG.  7    shows results of measuring the light sensitivity of the PD  20 A on a channel by channel basis by inputting light with a wavelength of 1.55 µm to the PLC  10  of the photonic integrated optical device  100 E through optical fiber, where the PD  20 A is optically coupled via the joint  30 D on the butt-joining end faces. Furthermore, subsequently, to evaluate bonding strength, shear testing was conducted by applying a force to the PD  20 A in a direction horizontal to the butt-joining end faces and results of the testing are shown. In addition, by way of comparison, using the PLC  10  on which the same substrate  19  was not mounted and PD  20 A, an photonic integrated optical device of a conventional configuration was produced by butt-joining the substrates  1  and  11  aligned with the layer surfaces of the core layers  3  and  13 , and results obtained from the conventional photonic integrated optical device are shown for the sake of comparison. 
     Referring to  FIG.  5   , based on the light sensitivity of the PD  20 A alone, there is little difference in the calculation results on the optical coupling loss and it can be seen that the photonic integrated optical device  100 E according to Embodiment 2 can implement butt-joining equivalent to that of the conventional photonic integrated optical device. In the shear testing conducted to evaluate the bonding strength, with the conventional photonic integrated optical device, a mode in which the PD  20 A and the UV-cure adhesive separated from each other occurred at 1.09 kgF. In contrast, with the photonic integrated optical device  100 E according to Embodiment 2, it was found that although a separation mode occurred at the same point as the conventional photonic integrated optical device, the force at which separation occurred was 2.85 kgF. As a result, it was found that the photonic integrated optical device  100 E exhibited improved bonding strength. These results confirmed that the photonic integrated optical device  100 E provides a stable connection structure. 
     As described above, with the photonic integrated optical device  100 E, in butt-joining the PLC  10  and the PD  20 A, the butt-joint holding substrate  19  that transmits UV light is used on the side of the optical circuit on top of the substrate  1 . Regarding the PD  20 A, the substrate  11  is used by being turned over such that the underside of the substrate  11  will be on the top side, and the UV-cure adhesive is filled into the butt-joined portion, irradiated with UV light, and thereby cured, forming the joint  30 D. That is, after the optical waveguides of the respective optical circuits are centered with respect to each other with the PLC  10  and the PD  20 A being butted together, in forming the joint  30 D by curing the UV-cure adhesive, the optical waveguides are butted together accurately to provide sufficient bonding strength in this structure. This makes it possible to implement hybrid integration of the PD  20 A simply and easily using the optical circuit of the PLC  10  as a platform and provide the photonic integrated optical device  100 E at low cost, where the photonic integrated optical device  100 E allows high accuracy butt-joining of optical waveguides. In particular, in the case of the photonic integrated optical device  100 E, the joint  30 D is formed after performing alignment using the alignment markers M provided near the butt-joining end faces of respective optical devices in addition to using the butt-joint holding substrate  19 . Consequently, the photonic integrated optical device  100 E allows optical coupling to be performed more stably and accurately than does the photonic integrated optical device  100 D according to Embodiment 1 and provides optical coupling characteristics with lower loss. 
     Embodiment 3 
       FIG.  8    is a perspective view showing a basic configuration of an photonic integrated optical device  100 F according to Embodiment 3 of the present invention as viewed obliquely from above. 
     Referring to  FIG.  8   , the photonic integrated optical device  100 F differs from the photonic integrated optical device  100 C in that a PD  20 B is used for the optical functional element  20 . Regarding the PLC  10 , whereas a light input unit 10 IN  and light output unit 10 OUT  for the core layer  3  are shown here, the substrate  19  is provided on top of the optical circuit here again. Regarding the PD  20 B, an input optical waveguide unit 20B IN  and a Ge-based photoelectric conversion unit  20 B a  are shown. That is, the photonic integrated optical device  100 F is generally the same as the photonic integrated optical device  100 E in terms of external configuration. Again, in the photonic integrated optical device  100 F, the UV-cure adhesive is filled into a gap between the optical circuit of the PD  20 B and the optical circuit of the PLC  10  and a gap between the end face of the substrate  11  of the PD  20 B and the end face of the substrate  19 . Consequently, the joint  30 D is formed with the UV-cure adhesive being cured. However, in the case of the photonic integrated optical device  100 F, markers applied to devices during production up to butt-joining differ from those of the other photonic integrated optical devices. As an example, the markers are metal markers such as aluminum markers formed by processing metal layers provided on top of the optical circuit of the PLC  10  and optical circuit of the PD  20 . Besides, a groove provided in a direction normal to the plane of the substrate  11  of the PD  20  is used for alignment, but this will be described later. 
     A Si substrate  1  measuring 5 mm in length, 10 mm in width, and 1 mm in board thickness was used for the silica-based PLC  10  making up the photonic integrated optical device  100 F. Then, a SiO 2  underclad layer  2  with a layer thickness of 20 µm, a SiO 2  core layer  3  with a core width of 4.5 µm and a film thickness of 4.5 µm, and a SiO 2  overclad layer  4  with a layer thickness of 15.5 µm were laminated on top of the substrate  1 . The optical circuit configured in this way may be called a PLC chip. Here, an optical waveguide was formed by the core layer  3  such that the core of the core layer  3  will differ by 2.0% in index of refraction from the overclad layer  4  and the underclad layer  2 . The PLC  10  receives input of light through the light input unit 10 IN  provided on a short side by being formed by the core layer  3  serving as the optical waveguide, and outputs the light through the light output unit 10 OUT  formed on a short side on the opposite side when viewed from the light input unit 10 IN  after the light has propagated through the core layer  3 . That is, four channels of the optical waveguide are installed with a pitch of 250 µm here and are each provided with an S-shaped structure running in a route from the light input unit 10 IN  to the photoelectric conversion unit  20 B a  of the PD  20 B. Whereas in the photonic integrated optical device  100 E, the alignment markers M are provided near the input/output end face of the core layers  3  and  13 , in the photonic integrated optical device  100 F, a metal marker MM formed of an Au pattern is provided on a surface of the PLC  10 . 
     A Si substrate  11  measuring 1.5 mm in length, 1.5 mm in width, and 0.625 mm in board thickness was used for the PD  20 B to be butt-joined to the PLC  10 . Then, a SiO 2  underclad layer  12  with a film thickness of 3.0 µm, a Si core layer  3  with a core width of 0.5 µm and a film thickness of 0.22 µm, and a SiO 2  overclad layer  14  with a film thickness of 1.5 µm were laminated on the underside of the substrate  11 . The optical circuit configured in this way may also be called a Si photonics chip. Here, again a spot size converter is used for the input optical waveguide unit 20B IN  and a mode field diameter (MFD: full width which makes intensity equal to 1/e 2  in a light intensity distribution) of the spot size converter is 3 µm both in vertical and horizontal directions of the PD  20 B. The light inputted through the input optical waveguide unit 20B IN  propagates through the core layer  13  and is converted into an electrical signal by the photoelectric conversion unit  20 B a . Light sensitivity of each light-receiving unit of the PD  20 B excluding optical coupling loss is 1.0 A/W at a wavelength of 1.55 µm. A metal marker MM formed of an Al pattern is provided on a surface of the PD  20 B in such a position as to align with the metal marker MM on the PLC  10  at the time of alignment with the PLC  10 . The reason why the Al metal marker MM is provided on the surface of the PD  20 B whereas the Au metal marker MM is provided on the surface of the PLC  10  is that the production process of Si generally uses Al rather than Au, which is expensive. 
     A groove V is provided in a direction normal to the surface of the PD  20 B, being exposed to an input end face of the PD  20 B. The groove V is deep enough to reach an inner region of the substrate  11  of the PD  20 B. The groove V coincides in position with a groove marker MV on the top face of the substrate  1  of the PLC  10  in a direction normal to the plane of the substrate  1 . This structure allows for alignment in a height direction. Note that the groove V is provided in such a way as to reach an upper region of the end face of the butt-joined substrate  1 . The depth of the groove V is adjusted by dry etching such that the height from the center of the core layer  3  of the PLC  10  to the top face of the overclad layer  4  will coincide with the depth of the center of the core layer  13  of the PD  20 B to the inner region of the substrate  11  at the bottom of the groove V. 
     The butt-joint holding substrate  19  measuring 5 mm in length, 2 mm in width, and 0.7 mm in board thickness is made of synthetic silica, which is a glass material, and is mounted and fixed to the top face of the PLC  10  with an adhesive or the like such that the butt-joining end face will be flush with the light output end face of the PLC  10 . 
     To butt-join the PD  20 B to the PLC  10  equipped with the substrate  19 , the substrate  1  of the PLC  10  and the substrate  11  of the PD  20 B are placed in a diagonal direction with respect to the layer surfaces of the respective core layers  3  and  13  of the PLC  10  and PD  20 B. Subsequently, the butt-joining end faces of the PLC  10  and the PD  20 B are prealigned by being observed obliquely from above the PLC  10 . Furthermore, to maximize the light sensitivity of the PD  20 B to the light outputted from the light output unit 10 OUT  of the PLC  10 , precise alignment is performed such that the position of the output light from the core layer  3  of the PLC  10  will accurately match the position of the input light to the core layer  13  of the PD  20 B. In so doing, alignment can be performed effectively if observation is carried out obliquely upward through the substrate  19 . 
     In Embodiment 3, the prealignment described above is performed using the metal markers MM provided on the surface of the PLC  10  and the surface of the PD  20 B.  FIGS.  9 ( a ) to  9 ( d )  are enlarged views showing a butt-joined portion of the photonic integrated optical device  100 F, where  FIG.  9 ( a )  is a top view of the butt-joined portion,  FIG.  9 ( b )  is a sectional side view taken along line IXb-IXb in  FIG.  9 ( a ) ,  FIG.  9 ( c )  is a plan view of the butt-joined portion during alignment using the groove and the groove marker MV, and  FIG.  9 ( d )  is a plan view of the butt-joined portion during prealignment using the metal markers MM. 
     The PLC  10 , in which the core layer  3  has a small difference in index of refraction from the underclad layer  2  and the overclad layer  4 , has a problem in that when the markers MM provided on the core layer  3  are observed, contrast of boundary surfaces between the core layer  3  and the underclad layer  2  and between the core layer  3  and the overclad layer  4  is low, resulting in a low resolution. Thus, as shown in  FIG.  9 ( a ) , when the metal markers MM are provided on the surface of the PLC  10 , a pattern of the metal markers MM can be checked more clearly. Thus, for more accurate alignment, desirably the metal markers are provided. 
     In the PD  20 B, if the metal markers are located near the input/output end face of the core layer  13 , the pattern of the metal markers MM in the depth direction of the core layer  13  can be checked transparently through the overclad layer  14 . However, if the metal markers are located on the surface of the PD  20 B, the pattern of the metal markers MM can be checked in an obliquely upward observation direction transparently through the combined thickness of the underclad layer  12  and overclad layer  14  as shown in  FIG.  9 ( b ) . This makes it possible to visually recognize the depth direction farther and easily check the pattern of the metal markers MM, and thus desirably the metal markers MM are provided also on the surface of the PD  20 B. If the butt-joining end face is observed obliquely from above, the pattern in the depth direction of the optical waveguide (core) in the core layer  13  can be observed transparently through the underclad layer  12  of the PD  20 B as shown in  FIG.  9 ( d ) . If a vernier pattern is used as a pattern of the metal markers MM and the scale width and vernier division are set to 10 µm and 0.5 µm, respectively, the metal markers MM are reduced to such a size as to be easy to observe through a camera. Consequently, along with the effect of providing the metal markers MM, prealignment can be performed with high accuracy. 
     If the metal markers MM are provided in two or more places near the butt-joining end faces and alignment adjustments are made such that the metal markers MM will align with each other, rotation runout around an axis vertical to the butt-joining end faces can also be corrected. In this way, if the metal markers MM for alignment are provided on the butt-joining end faces of the PLC  10  and PD  20 B, this is effective in improving alignment accuracy. 
     In addition, according to Embodiment 3, as shown in  FIG.  9 ( c ) , the use of the groove provided in the PD  20 B and the groove marker MV provided in the PLC  10  makes it possible to perform alignment accurately in directions (height direction) normal to the planes of the substrates  1  and  11  as well. If alignment is performed such that the bottom face of the groove V in the PD  20 B will align with the groove marker MV provided in a surface of the overclad layer  4  of the PLC  10 , by passing through the substrate  19  obliquely from above, alignment in a direction normal to the butt-joining end faces of the substrates  1  and  11  can be performed in accordance with designed depth of the groove V. 
     Whereas the depth of the groove in the PD  20 B is set such that the groove will align with the surface of the overclad layer  4  of the PLC  10 , it is sufficient that the pattern is visually recognizable by being set to contact the input/output end face of the PLC  10 . That is, it is presupposed that just after the prealignment, the core layers  3  and  13  of the PLC  10  and PD  20 B are generally aligned in position with each other. Besides, regarding the depth of the groove in the PD  20 B, it is sufficient that the distance from the center of the core layer  13  of the PD  20 B to the bottom face of the groove V in the direction normal to the plane of the substrate  11  is set so as to coincide with the distance from the center of the core layer  3  of the PLC  10  to the pattern provided on the surface of the PLC  10  in the direction normal to the plane of the substrate  1 . 
     To improve visibility, desirably the groove V runs to the butt-joining end face of the PD  20 B. Secondary effects of this arrangement include the effect of keeping the UV-cure adhesive introduced into a gap between end faces from forming a large adhesive fillet at a connection on the surface of the PD  20 B. In this way, the photonic integrated optical device  100 F was produced. 
       FIG.  10    is a diagram showing evaluation results on bonding strength as measured by optical coupling loss [dB] in the photonic integrated optical device  100 F, where the loss is measured by applying a force to a PD  20 B in a direction horizontal to butt-joining end faces after light sensitivity of the PD  20 B is measured on a channel by channel basis. However,  FIG.  10   , shows results of measuring the light sensitivity of the PD  20 B on a channel by channel basis by inputting light with a wavelength of 1.55 µm to the PLC  10  of the photonic integrated optical device  100 E through optical fiber, where the PD  20 B is optically coupled via the joint  30 D on the butt-joining end faces. Furthermore, subsequently, to evaluate bonding strength, shear testing was conducted by applying a force to the PD  20 B in a direction horizontal to the butt-joining end faces and results of the testing are shown. In addition, by way of comparison, using the PLC  10  and PD  20 B on which the same substrate  19  was not mounted, an photonic integrated optical device of a conventional configuration was produced by butt-joining the substrates  1  and  11  aligned with the layer surfaces of the core layers  3  and  13 , and results obtained from the conventional photonic integrated optical device are shown for the sake of comparison. 
     Referring to  FIG.  10   , based on the light sensitivity of the PD  20 B alone, there is little difference in the calculation results on the optical coupling loss and it can be seen that the photonic integrated optical device  100 F according to Embodiment 3 can implement butt-joining equivalent to that of the conventional photonic integrated optical device. In the shear testing conducted to evaluate the bonding strength, with the conventional photonic integrated optical device, a mode in which the PD  20 B and the UV-cure adhesive separated from each other occurred at 1.10 kgF. In contrast, with the photonic integrated optical device  100 F according to Embodiment 3, it was found that although a separation mode occurred at the same point as the conventional photonic integrated optical device, the force at which separation occurred was 2.90 kgF. As a result, it was found that the photonic integrated optical device  100 F exhibited improved bonding strength. These results confirmed that the photonic integrated optical device  100 F provides a stable connection structure. 
     That is, with the photonic integrated optical device  100 F, the substrate  19  is used for butt-joining to the end face of the PD  20 B by being mounted on top of the substrate  1  of the PLC  10 . Besides, during butt-joining, the substrate  1  of the PLC  10  and the substrate  11  of the PD  20 B are placed diagonally with respect to the layer surfaces of the optical waveguides (core layers  3  and  13 ) of the respective optical circuits of the PLC  10  and the PD  20 B. The metal markers MM and the groove marker MV are used for alignment between the butt-joining end faces of the PLC  10  and PD  20 B with the substrate  19  being mounted on the PLC  10 . Furthermore, after the alignment and subsequent centering that is performed using externally introduced light, the joint  30 D is formed with the UV-cure adhesive being cured. 
     As described above, with the photonic integrated optical device  100 F, in butt-joining the PLC  10  and the PD  20 B, the butt-joint holding substrate  19  that transmits UV light is used on the side of the optical circuit of top of the substrate  1 . Regarding the PD  20 B, the substrate  11  is used by being turned over such that the underside of the substrate  11  will be on the top side, and the UV-cure adhesive is filled into a gap between the butted portions, irradiated with UV light, and thereby cured, forming the joint  30 D. That is, after the optical waveguides of the respective optical circuits are centered with respect to each other with the PLC  10  and the PD  20 B being butted together, in forming the joint  30 D by curing the UV-cure adhesive, the optical waveguides are butted together accurately to provide sufficient bonding strength in this structure. This makes it possible to implement hybrid integration of the PD  20 B simply and easily using the optical circuit of the PLC  10  as a platform and provide the photonic integrated optical device  100 F at low cost, where the photonic integrated optical device  100 F allows high accuracy butt-joining of optical waveguides. In particular, in the case of the photonic integrated optical device  100 F, prealignment is performed using the alignment metal markers MM provided on a surface near the butt-joining end faces of respective optical devices in addition to using the butt-joint holding substrate  19 . Subsequently, the joint  30 D is formed by performing alignment in the height direction using the groove V provided in the PD  20 B and the groove marker MV facing the groove V by being provided in the PLC  10 . Consequently, the photonic integrated optical device  100 F allows optical coupling to be performed still more stably and accurately than does the photonic integrated optical device  100 E according to Embodiment 2 and provides optical coupling characteristics with lower loss.