Patent Publication Number: US-2010111468-A1

Title: Optical integrated circuit and optical integrated circuit module

Description:
The present application is a Continuation-in Part application of U.S. patent application Ser. No. 12/045,281, filed Mar. 10, 2008, the entire contents of which being incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The present invention relates to an optical integrated circuit formed by interconnecting a plurality of optical components, and more specifically, relates to an optical circuit and an optical integrated circuit module. In the optical integrated circuit module, optical hybrid integrated devices, in which a planar lightwave circuit having optical waveguides formed on a PLC platform, and a semiconductor device having optical active components, such as semiconductor laser diodes and semiconductor photodiodes formed on a semiconductor substrate are coupled with each other. 
     2) Description of the Related Art 
     With the spread and progress of optical communication networks, functions of optical components for use in optical transmission systems have been sophisticated. The optical components include optical active components for emitting or receiving optical signals, optical passive components for splitting/coupling or demultiplexing/multiplexing the optical signals, optical fibers for use in transmission lines of the optical signals, or the like, and an improvement in performance or a reduction in cost is increasingly required for respective optical components. Among these, with regard to the optical active components, devices based on semiconductor materials, such as semiconductor lasers and semiconductor photodiodes are the main devices, and technical development thereof has been advanced. The optical active component based on the semiconductor material has features of allowing optical amplification function, high-speed operation, and compact integration. Meanwhile, with regard to the optical passive components, planar lightwave circuits (PLC; Planar Lightwave Circuit, which will be referred to PLC hereinbelow) having optical waveguides based on silica-based materials are commercially produced. PLC has advantageous features of allowing optical waveguides to realize with low loss and without polarization dependency. 
     While improvement in performance of respective elements has been independently made for both the optical active element and the optical passive element until now, requirement for high performance optical components having both advantages has been increased because of sophisticated needs resulting from development of the optical transmission systems. Therefore, developments of optical hybrid integrated devices in which semiconductor active elements (optical active component) such as semiconductor laser diodes or the like, and PLC are combined with each other have been made. 
     In a conventional art disclosed in, for example, Patent Document 1, a semiconductor laser diode is hybrid-mounted on the PLC platform, and thus achieving a laser that oscillates in an external resonator mode which is formed between the semiconductor laser diode and a UV grating on the PLC. In this conventional art, there is only one waveguide for introducing a light outputted from the semiconductor laser diode into the UV grating. Therefore, one end facet of the emitting waveguide (a semiconductor waveguide on the Si terrace for mounting the laser diode) of the semiconductor laser diode and one end facet of the optical waveguide on the PLC are coupled with each other. 
     Document 1: Japanese Unexamined Patent Publication (Kokai) No. 2001-267684 
     Meanwhile, in a conventional art disclosed in following Document 2, an optical wavelength selector is achieved by hybrid-integrating an arrayed waveguide grating (AWG) on the PLC and semiconductor optical amplifiers (SOAs). Here, SOAs are used as gate switches, wherein input waveguides and output waveguides of SOAs are in contact with different end facets of the semiconductor substrate, and in contact with the PLC platform at respective end facets to optically couple with the optical waveguides on the PLC platform. 
     Document 2: I. Ogawa, F. Ebisawa, N. Yoshimoto, K. Takiguchi, F. Hanawa, T. Hashimoto, A. Sugita, M. Yanagisawa, Y. Inoue, Y. Yamada, Y. Tohmori, S. Mino, T. Ito, K. Magari, Y. Kawaguchi, A. Himeno, and K. Kato, “Lossless hybrid integrated 8-ch optical wavelength selector module using PLC platform and PLC-PLC direct attachment techniques” Proc. OFC &#39;98, 1998, paper PD4-1 
     Moreover, in following Document 3, there is disclosed a technology in which waveguides on different PLC platforms (a first PLC platform and a second PLC platform) are optically coupled with each other. In this conventional art, the waveguides on one PLC has a turnaround portion. However, since it is difficult to achieve a high refractive index difference in PLC, there is no choice other than setting a radius of curvature of the bent waveguide in the turnaround portion to a quite large value. 
     Document 3: Japanese Unexamined Patent Publication (Kokai) No. H10-227936 
     However, when the semiconductor element, such as a SOA having the input waveguide and the output waveguide, and the optical waveguides on right and left PLCs existing on both sides of the semiconductor element are coupled with each other as the conventional art disclosed in aforementioned Document 2, following fixing is required. Namely, one end facet of the semiconductor substrate with the end of the input waveguides is fixed to the end facet of one PLC platform, and the other end facet of the semiconductor substrate with the end of the output waveguides is also fixed to the end facet of the other PLC platform. As a result, the input waveguides of the semiconductor elements are coupled with the optical waveguides of one PLC, and the output waveguides thereof are coupled with the optical waveguides of the other PLC. 
     As described above, the number of surfaces (contact surfaces) for fixing the semiconductor substrate which has the input waveguides and the output waveguides and on which the semiconductor elements is formed, and the PLC platform is increased. In this case, it is necessary to obtain excellent couplings between the waveguide on the semiconductor substrate and the optical waveguides on the right and left PLC platforms at respective contact surfaces. For this reason, in the conventional art described in aforementioned Document 2, optical alignment works between the waveguides must be performed at two contact surfaces. One of two contact surfaces is a contact surface between one end facet of the semiconductor substrate and the end facet of one PLC platform, and another is a contact surface between the other end facet of the semiconductor substrate and the end facet of the other PLC platform, respectively. Hence, since the man-hour for alignment increases in this conventional art, the optical alignment works will be troublesome and take time, and a possibility that alignment mistakes may occur will also be increased. As a result, there has been a problem of difficulty in obtaining the excellent coupling efficiency. 
     Meanwhile, it is conceivable to insert the semiconductor elements into an area (cutout portion) where a part of the optical waveguides on the one PLC platform is cut off, and then arrange it. However, even in this case, there have been problems that a dimensional accuracy to a length of the cutout portion or the semiconductor element would be severe, or the optical alignment works would be complicated and difficult, in order to make the coupling efficiency between the input side and the output side waveguides of the semiconductor element, and the optical waveguides on the PLC platform excellent. 
     Further, when those hybrid-integrated circuit of the semiconductor element and PLC platforms is modularized with fiber pigtails or fiber arrays, the optical alignment works between the PLC platforms and the fibers need to be performed at two points of the end facets of the PLC platforms and the man-hour for alignment increases by that much. As a result, the optical alignment works will be troublesome and time consuming, and the possibility that optical alignment mistakes may occur will also be increased, thus causing the problem of difficulty in obtaining the excellent coupling efficiency. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of the above-mentioned conventional problems. The present invention has an object provide a compact optical integrated circuit and a compact optical integrated circuit module of a planar lightwave circuit and a semiconductor element, in which optical alignment works are easily performed and excellent coupling efficiency is easily obtained. 
     An optical integrated circuit in accordance with a first aspect of the present invention is provided with a planar lightwave circuit in which an optical waveguide is formed on a first substrate; and a semiconductor element in which at least one element having a semiconductor waveguide is formed on a second substrate, and a turnaround waveguide which is turned around on the second substrate and is connected to an input port or an output port of the element having said semiconductor waveguide, wherein the planar lightwave circuit and the semiconductor element are fixed at one contact surface, and an input port and output port of the turnaround waveguide an end of the optical waveguide and an end of the semiconductor waveguide are optically coupled with each other at the contact surface with an input port and an output port of the optical waveguide. 
     According to this aspect, since the contact surface between the planar lightwave circuit and the semiconductor element, namely, the contact surface between the first substrate of the planar lightwave circuit and the second substrate of the semiconductor element results in only one contact surface, optical alignment works for coupling both of them can be performed at once. For this reason, the man-hour for alignment can be reduced, the optical alignment works can be easily performed, and a possibility that alignment mistakes may occur will also be reduced, thereby allowing excellent coupling efficiency to be obtained. Additionally, since input/output fibers may also be in contact with the planar lightwave circuit only at an end facet of one side thereof, it is also possible to reduce optical alignment works of this portion. Further, there are also advantages that strict dimensional accuracy against a length of the semiconductor element or the like is not required, either. 
     The “planar lightwave circuit (PLC)” described here means a circuit in which the optical waveguide is formed with materials of a quartz system or a polymer system on the substrate of silicon or quartz by combining optical fiber manufacturing technologies and semiconductor microfabrication technologies. 
     In the optical integrated circuit in accordance with a second aspect of the present invention, an input semiconductor waveguide and an output semiconductor waveguide are formed at an input side and an output side of the semiconductor element, respectively, one of the input and the output semiconductor waveguides has a turnaround portion turned around on the second substrate, and for alignment purposes an end of the input semiconductor waveguide and an end of the output semiconductor waveguide are optically coupled with an end of the input side optical waveguide and an end of the output side optical waveguide formed on the first substrate at the contact surface, respectively. 
     A refractive index difference between a core and a clad composing this optical waveguide is typically less than or comparable to several percents in the optical waveguide of the planar lightwave circuit of a normal quartz system. The refractive index difference between the core and the clad composing this optical waveguide may be set to a large value of more than 10% in the semiconductor waveguide. The larger the refractive index difference between the core and the clad, the smaller the radius of curvature of the turnaround portion at the time of turning around the waveguide (bent waveguide) can be made. For that reason, fabricating the turnaround waveguide on the semiconductor makes it possible to greatly reduce the size of the element as compared with a case where the turnaround waveguide is fabricated on the planar lightwave circuit of the quartz system. 
     According to the second aspect, while one of the input and output semiconductor waveguides of the element has the turnaround portion turned around on the second substrate, the radius of curvature of the turnaround portion can be reduced in the semiconductor waveguide, thus allowing the size of the semiconductor element to be greatly reduced. Hence, the compact optical integrated circuit in which the planar lightwave circuit and the semiconductor device are integrated can be achieved. 
     In contrast to this, the conventional art disclosed in aforementioned Document 3 has a configuration in which the waveguides on the first PLC platform and the second PLC platform are coupled with each other, and the waveguide is turned around on any one of the PLC platforms. Since it is difficult to achieve the waveguide with high refractive index difference in the optical waveguide on the PLC quartz system as compared with the semiconductor waveguide, there is no choice other than setting the radius of curvature of the turnaround portion of the bent waveguide to a quite large value. 
     Moreover, according to the second aspect, the end of the input semiconductor waveguide and the end of the output semiconductor waveguide of the element are coupled with the different optical waveguides at the contact surface. Hence, the contact surface between the semiconductor element and the planar lightwave circuit results in only one contact surface, although there are the input and the output semiconductor waveguides of the element. For this reason, the optical alignment works for coupling the optical waveguides on the planar lightwave circuit and the semiconductor waveguides on the semiconductor element can be performed at once. 
     Note herein that, the “element” described herein includes for example, Semiconductor Optical Amplifiers (SOA), Electro Absorption (=EA) modulators using the electric field absorption effect of the semiconductor, semiconductor lasers, semiconductor photo detectors, or the like. 
     In the optical integrated circuit in accordance with a third aspect of the present invention, a plurality of elements having the semiconductor waveguides are arranged in array pattern. 
     According to this aspect, even when the optical integrated circuit is fabricated by integrating the semiconductor device in which a plurality of elements are arranged in array pattern, and the planar lightwave circuit, the optical alignment works are easily performed and excellent coupling efficiency can also be obtained. 
     In the optical integrated circuit in accordance with a fourth aspect of the present invention, ends of all the semiconductor waveguides formed on the second substrate and ends of all the optical waveguides formed on the first substrate are optically coupled with each other at the contact surface. 
     According to this aspect, although there are many waveguides coupled at the contact surface, the planar lightwave circuit and the semiconductor device are made contact with each other at one contact surface to be fixed, so that it is possible to perform the alignment and fixing works of the planar lightwave circuit and the semiconductor device at once. 
     In the optical integrated circuit in accordance with a fifth aspect of the present invention, RF electrodes for supplying RF signals to the elements are formed on the second substrate. 
     In the optical integrated circuit in accordance with a sixth aspect of the present invention, the element having the semiconductor waveguide is a semiconductor light receiving element in which the input semiconductor waveguide is formed only on the input side thereof, the end of the input semiconductor waveguide is optically coupled with the optical waveguide at the contact surface, a first optical waveguide for alignment and a second optical waveguide for alignment are formed on the first substrate for guiding a light for alignment, a turnaround waveguide for alignment is formed on the second substrate, and a light emitting end of the first optical waveguide for alignment and a light incident end of the second optical waveguide for alignment are optically coupled with a light incident end and a light emitting end of the turnaround waveguide for alignment at the contact surface, respectively. 
     In the optical integrated circuit in accordance with a seventh aspect of the present invention, the elements are arranged in array pattern. 
     In the optical integrated circuit in accordance with an eighth aspect of the present invention, the element having the semiconductor waveguide is a semiconductor light emitting element in which the output semiconductor waveguide is formed only on the output side, and the end of the output semiconductor waveguide is optically coupled with the end of the optical waveguide at the contact surface. 
     An optical integrated circuit module in accordance with the present invention is provided with the above-mentioned optical integrated circuit, and optical fibers for input/output arranged at an end facet opposite to the contact surface of the first substrate, wherein ends of the optical fibers for input/output are optically coupled with the optical waveguides on the first substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken into connection with the accompanying drawing wherein one example is illustrated by way of example, in which; 
         FIG. 1  is a perspective view showing a basic configuration of an optical integrated circuit in accordance with a first embodiment; 
         FIG. 2  is a plan view showing the optical integrated circuit in accordance with the first embodiment; 
         FIG. 3  is a sectional view along a line A-A′ shown in  FIG. 2 ; 
         FIG. 4  is a sectional view along a line B-B′ line shown in  FIG. 2 ; 
         FIG. 5  is a perspective view showing a schematic configuration of an optical integrated circuit in accordance with a second embodiment; 
         FIG. 6  is a plan view showing the optical integrated circuit in accordance with the second embodiment; 
         FIG. 7  is a perspective view showing a schematic configuration of an optical integrated circuit in accordance with a third embodiment; 
         FIG. 8  is a perspective view showing a schematic configuration of an optical integrated circuit in accordance with a fourth embodiment; and 
         FIG. 9  is a plan view showing a schematic configuration of an optical integrated circuit in accordance with a fifth embodiment; 
         FIG. 10  is a plan view showing the optical integrated circuit in accordance with the sixth embodiment; 
         FIG. 11  is a plan view showing the optical integrated circuit in accordance with the seventh embodiment; 
         FIG. 12  is a plan view showing the optical integrated circuit in accordance with the eighth embodiment; 
         FIG. 13  is a plan view showing the optical integrated circuit in accordance with the ninth embodiment; 
         FIG. 14  is a sectional view along a line C-C shown in  FIG. 10 , and shows a cross-sectional structure of the optical integrated circuit in accordance with the tenth embodiment; and 
         FIG. 15  is a sectional view along a line D-D shown in  FIG. 10 ; 
         FIG. 16  is a sectional view showing an optical integrated circuit for explaining a method for mounting of the first embodiment; 
         FIG. 17  is a plan view showing an optical integrated circuit for explaining a method for mounting of the first embodiment; 
         FIG. 18  is a sectional view showing an optical integrated circuit for explaining a method for mounting of the second embodiment; 
         FIG. 19  is a plan view showing an optical integrated circuit for explaining a method for mounting of the second embodiment; and 
         FIG. 20  is a sectional view showing an optical integrated circuit having a reverse connecting structure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, each of embodiments of an optical integrated circuit and an optical integrated circuit module that embody the present invention will be described based on the drawings. Incidentally, in the description of each embodiment, the same reference numeral is given to a similar part and a duplicated description will be omitted. 
     First Embodiment 
     An optical integrated circuit in accordance with a first embodiment of the present invention will be described based on  FIG. 1  through  FIG. 4 .  FIG. 1  is a conceptual diagram showing a basic configuration of the optical integrated circuit in accordance with the first embodiment, while  FIG. 2  is a plan view showing the same optical integrated circuit.  FIG. 3  is a sectional view along a line A-A′ shown in  FIG. 2 , and shows a cross-sectional structure of the planar lightwave circuit. Meanwhile,  FIG. 4  is a sectional view along a line B-B′ shown in  FIG. 2 , and shows a cross-sectional structure of a semiconductor waveguide portion of a semiconductor element. 
     An optical integrated circuit  1  is a circuit in which a planar lightwave circuit (PLC)  2  and a semiconductor element  3  fixed on a silicon substrate  7  are integrated as shown in  FIG. 1  and  FIG. 2 . 
     The planar lightwave circuit  2  is provided with a PLC platform  4  and two straight optical waveguides  5  and  6  formed on the PLC platform  4 . The optical waveguides  5  and  6  are extended from one end facet  2   a  to the other end facet  2   b  of the planar lightwave circuit  2 , respectively. Namely, one ends of the optical waveguides  5  and  6  are in contact with one end facet (left side end facet in  FIG. 1 ) of the PLC platform  4 , respectively, and the other ends thereof are in contact with the other end facet (right side end facet in  FIG. 1 ) of the PLC platform  4 , respectively. The PLC platform  4  is a silicon substrate, for example. 
     The semiconductor element  3  is provided with a semiconductor substrate  8 , and a semiconductor optical amplifier (SOA)  9  as an element formed on this semiconductor substrate  8  as shown in  FIG. 1  and  FIG. 2 . An input semiconductor waveguide  10  and an output semiconductor waveguide  11  are further formed on the semiconductor substrate  8  at an input side and an output side of the semiconductor amplifier  9 , respectively. The output semiconductor waveguide  11  has a turnaround portion  11   a  turned around on the semiconductor substrate  8  where a propagating direction of a light is turned around, and is in contact with an end facet  3   a  of the semiconductor element  3  on the same side as the input semiconductor waveguide  10 . 
     The optical integrated circuit  1  is characterized by following configurations. 
     The planar lightwave circuit  2  and the semiconductor element  3  are fixed at one contact surface  12 . Namely, the other end facet  2   b  of the planar lightwave circuit  2  and the end facet  3   a  of the semiconductor element  3  are fixed. 
     The element formed on the semiconductor substrate  8  is the semiconductor optical amplifier (SOA)  9 . 
     The input semiconductor waveguide  10  and the output semiconductor waveguide  11  are formed on the input side and the output side of the semiconductor amplifier  9 , respectively. The output semiconductor waveguide  11  has the turnaround portion  11   a  turned around on the semiconductor substrate  8 . 
     Respective ends of the optical waveguides  5  and  6  and respective ends of semiconductor waveguides  10  and  11  are coupled with each other on one contact surface  12 . Namely, the end of the optical waveguide  5  and the end of the optical waveguide  6  are coupled with the end of the input semiconductor waveguide  10  and the end of the output semiconductor waveguide  11  on one contact surface  12 , respectively. 
     The planar lightwave circuit  2  is composed of the PLC platform  4 , a lower clad layer  14  formed on the PLC platform  4 , core layers  15  and  16  formed on the lower clad layer  14 , and an upper clad layer  17  formed on the lower clad layer  14  and the core layers  15  and  16 , as shown in  FIG. 3 . In the planar lightwave circuit  2  composed as above, the optical waveguides  5  and  6  is composed of the core layers  15  and  16  with high refractive index serving as paths of the light, and the clad layers  14  and  17  with low refractive index, which are peripheries thereof. As an example, the optical waveguides  5  and  6  are quartz glass waveguides in which the lower clad layer  14 , the core layers  15  and  16 , and the upper clad layer  17  are formed with quartz system materials in the present embodiment. In the optical waveguides  5  and  6  described above, a refractive index difference between the core layers  15  and  16 , and the clad layers  14  and  17  is typically less than or comparable to several percents. 
     In the optical integrated circuit depicted in  FIG. 1 , an output semiconductor waveguide  11  having a turnaround portion  11   a  turned on the semiconductor substrate  8  is a turnaround waveguide  90  connected to an output port of the semiconductor optical amplifier (SOA)  9  as the element. The present invention is not limited to the aforementioned configuration. The present invention is applicable to the configuration in which the turnaround portion is formed in the side of the input semiconductor waveguide  10 , and the input semiconductor waveguide  10  is the turnaround waveguide connected to the input port of the SOA  9 . The same can be applied to the turnaround waveguide  90  described hereunder in each of the following embodiments. 
     Furthermore, although the turnaround waveguide  90  is directly connected to the output port of the SOA  9  in the optical integrated circuit depicted in  FIG. 1 , the present invention is applicable to the case in which other waveguide or branch waveguide is arranged between the turnaround waveguide  90  and the input port or the output port of the semiconductor optical amplifier (SOA)  9 . The same can be applied to the turnaround waveguide  90  described hereunder in each of the following embodiments. 
     The aforementioned planar lightwave circuit  2  is formed by following methods. Glass particles to be the lower clad layer  14  and the core layers  15  and  16  are deposited on the PLC platform (for example, silicon substrate)  4  by a flame hydrolysis deposition (FHD) method which is an application of optical fiber fabrication technologies, and are melted by heating to make a glass membrane transparent. Subsequently, a desired optical waveguide pattern is formed by photolithography and reactive ion etching (RIE), which are semiconductor integrated circuit manufacturing technologies, and the upper clad layer  17  is formed by the FHD method again. 
     The input semiconductor waveguide  10  and the output semiconductor waveguide  11  formed on the semiconductor substrate  8  are provided with lower clad layers  20  formed on the semiconductor substrate  8 , core layers  21  formed on the lower clad layers  20 , and upper clad layers  22  formed on the core layers  21 , respectively, as shown in  FIG. 4 . The semiconductor substrate  8  is formed of a compound semiconductor InP; the lower clad layer  20 , a compound semiconductor InP; the core layer  21 , compound semiconductor InGaAsP; and the upper clad layer  22 , a compound semiconductor InP, respectively. Additionally, the semiconductor waveguide  10  is a straight waveguide formed into a high mesa structure. The semiconductor waveguide  11  is a waveguide, which is formed into a high mesa structure and has the turnaround portion  11   a.  The semiconductor waveguides may have an embedded structure and a low mesa structure. In the case of forming the semiconductor waveguides into the high mesa structure as the present example, a refractive index difference between the core layer  21  and air on both sides is significantly large, for example, 40% or more. Therefore, low loss can be maintained even when a radius of curvature of the turnaround portion  11   a  is decreased. 
     The semiconductor optical amplifier  9  formed on the semiconductor substrate  8  differs in a configuration from the semiconductor waveguides  10  and  11  in that the core layer of the semiconductor waveguides  10  and  11  is an active layer  23  formed by an optical amplification medium. The semiconductor optical amplifier  9  and the semiconductor waveguides  10  and  11  are then formed on the semiconductor substrate  8  so that the light transmitted within the core layer  21  of the semiconductor waveguide  10  may pass through the active layer  23  of the semiconductor optical amplifier  9  and the core layer  21  of the semiconductor waveguide  11 . In the present embodiment, the semiconductor optical amplifier  9  is used as a semiconductor gate in which an incident light is turned on and off by turning on and off an injection current. The optical integrated circuit  1  having the aforementioned configuration is fabricated as follows. 
     The planar lightwave circuit  2  and the semiconductor element  3  are made contact with each other at one contact surface  12 . Namely, the other end facet  2   b  of the planar lightwave circuit  2  and the end facet  3   a  of the semiconductor element  3  are made contact with each other. In this state, an optical alignment between the optical waveguide  5  and the input semiconductor waveguide  10  and an optical alignment between the optical waveguide  6  and the output semiconductor waveguide  11  are performed. An active alignment is employed as the optical alignment method, in which a light for alignment is entered into the optical waveguide  5  from an incident port  5   a  side of the optical waveguide  5  in a state where currents are made to flow through the semiconductor optical amplifier  9  on the semiconductor substrate  8 , a light which has passed through the semiconductor waveguide  10 , the semiconductor optical amplifier  9 , the semiconductor waveguide  11 , and the optical waveguide  6 , and emitted from an emitting port  6   a  is received by a light receiving element (not shown), and alignment between the planar lightwave circuit  2  and the semiconductor element  3  is performed so that the amount of light to be received may be the maximum level. 
     Note herein that, although the alignment by the active alignment is performed in the present embodiment, it is also possible to perform passive alignment by utilizing position markers, concavo-convex shapes for alignment, or the like formed on the PLC platform  4  and the semiconductor substrate  8 . 
     Since a thickness of the semiconductor substrate  8  is thin, compared with that of the PLC platform  4 , the semiconductor element  3  on the semiconductor substrate  8  is fixed on the silicon substrate  7 , and the PLC platform  4  and the silicon substrate  7  are then attached, so that sufficient attachment strength is ensured in the present embodiment. 
     Meanwhile, the spot size of the optical waveguide on the PLC generally differs from that of the semiconductor waveguide. A structure for converting the spot size is provided in the portion where both of the waveguides are coupled with each other to thereby adjust the spot sizes of the optical waveguide and the semiconductor waveguide, thus allowing further higher coupling efficiency to be obtained. 
     According to the first embodiment having the above configuration, following functions and effects can be obtained. 
     The contact surface  12  between the planar lightwave circuit  2  and the semiconductor element  3 , namely, the contact surface between the PLC platform (first substrate)  4  of the planar lightwave circuit  2  and the semiconductor substrate  8  (second substrate) of the semiconductor element  3  results in only one contact surface. By this construction, optical alignment works for coupling both of them can be performed at once. For this reason, the man-hour for alignment can be reduced, the optical alignment works can be easily performed, and a possibility that alignment mistakes may occur will also be reduced, thereby allowing excellent coupling efficiency to be obtained. Hence, it is possible to achieve the compact optical integrated circuit  1  in which the optical alignment works can be easily performed and the excellent coupling efficiency can be easily obtained, and the planar lightwave circuit  2  and the semiconductor device  3  are integrated. 
     Since the planar lightwave circuit  2  and the semiconductor element  3  are fixed at one contact surface  12 , the alignment and fixing works of the planar lightwave circuit  2  and the semiconductor element  3  can also be performed at once, so that it is advantageous in the viewpoint of a reduction in fabrication time and a cost reduction. 
     The end of the input semiconductor waveguide  10  and the end of the output semiconductor waveguide  11  of the semiconductor optical amplifier (element)  9  are coupled with the different optical waveguides  5  and  6  at the contact surface  12 . By this construction, the contact surface  12  between the semiconductor element  3  and the planar lightwave circuit  2  results in only one contact surface, although there are the input and the output semiconductor waveguides of the semiconductor optical amplifier  9 , respectively. For this reason, the optical alignment works for coupling the optical waveguides  5  and  6  of the planar lightwave circuit  2 , and the semiconductor waveguides  10  and  11  of the semiconductor element  3  can be performed at once. 
     Since input/output fibers are also in contact with the planar lightwave circuit  2  only at the end facet (one end facet  2   a  of the planar lightwave circuit  2 ) on one side, it is also possible to reduce the optical alignment works of this portion. 
     Since the semiconductor element  3  is fixed with the other end facet  2   b  of the planar lightwave circuit  2  only at the end facet  3   a  of one side thereof, an end facet  3   b  opposite to the end facet  3   a  of the semiconductor element  3  is free. For this reason, strict dimensional accuracy against a length of the semiconductor element  3  or the like is not required, either. Hence, fabrication of the semiconductor element  3  becomes easy. 
     The output semiconductor waveguide  11  has the high mesa structure and the refractive index difference between the core and the clad is as very large as 40% or more. By this construction, it is possible to suppress the loss to a low level even when the semiconductor waveguide  11  is turned around with a very small radius of curvature (for example, radius of curvature of about  125  micrometers). Since the output semiconductor waveguide  11  of the semiconductor optical amplifier  9  has the turnaround portion  11   a  turned around on the semiconductor substrate  8 , the radius of curvature of the turnaround portion  11   a  can be reduced, thus allowing a size of the semiconductor element  3  to be greatly reduced. Hence, the compact optical integrated circuit in which the planar lightwave circuit  2  and the semiconductor element  3  are integrated can be achieved. 
     Anti-reflection coating for suppressing a reflection at the end facet to a low level is often applied to the end facet of the semiconductor waveguide. The coating is required for respective end facets ( 3   a  and  3   b  shown in  FIG. 2 ) on the input side and the output side if the waveguide is not turned around. In the present embodiment, both of the ends of the input and output waveguides face to the same end facet  3   a  by turning around the waveguide. And thus, the anti-reflection coating may also be applied only to one side (only the end facet  3   a ), resulting in simple fabrication steps of the semiconductor element. 
     When the optical coupling from the PLC waveguide to the input semiconductor waveguide is not optimal, an uncoupled light may reach the end facet  3   b  on the opposite side thereof as a stray light depending on the structure of the semiconductor waveguide to thereby be mixed into the output side waveguide. In the present embodiment, the output semiconductor waveguide  11  has the turnaround portion  11   a,  the output side waveguide faces to the same end facet  3   a  as the input waveguide. Therefore, a stray light component is hard to return to the end facet  3   a,  and thus it is hard for the stray light to be mixed into the output waveguide. 
     Second Embodiment 
     Next, an optical integrated circuit in accordance with a second embodiment will be described based on  FIG. 5  and  FIG. 6 .  FIG. 5  is a conceptual diagram showing a schematic configuration of an optical integrated circuit  1 A in accordance with a second embodiment, while  FIG. 6  is a plan view showing the optical integrated circuit  1 A. 
     The optical integrated circuit  1 A is characterized in that, in the optical integrated circuit  1  in accordance with the aforementioned first embodiment shown in  FIG. 1 , a plurality of semiconductor optical amplifiers (elements) are arranged in array pattern on the semiconductor substrate  8  of a semiconductor element  3 A. As an example, four semiconductor optical amplifiers  9   1  to  9   4  are arranged in array pattern on the semiconductor substrate  8  as shown in  FIG. 5  and  FIG. 6  in the present embodiment. 
     Meanwhile, there are formed on the semiconductor substrate  8  the input semiconductor waveguides  10   1  to  10   4  and the output semiconductor waveguides  11   1  to  11   4  on the input side of respective semiconductor optical amplifiers  9   1  to  9   4  and on the output side of respective semiconductor optical amplifiers  9   1  to  9   4 , respectively. The output semiconductor waveguides  11   1  to  11   4  have the turnaround portions  11   a  turned around on the semiconductor substrate  8 , respectively. All the semiconductor waveguides  10   1  to  10   4  and  11   1  to  11   4  on the semiconductor substrate  8  are in contact with one end facet  3   a  of the semiconductor element  3 . 
     Additionally, while using two straight optical waveguides  5  and  6  as one set, four sets of optical waveguides  5   1  and  6   1  to  5   4  and  6   4  are formed on the PLC platform  4  of a planar lightwave circuit  2 A corresponding to all the semiconductor waveguides  10   1  to  10   4  and  11   1  to  11   4  on the semiconductor substrate  8 , respectively. Four sets of optical waveguides  5   1  and  6   1  to  5   4  and  6   4  are extended from one end facet  2   a  to the other end facet  2   b  of the planar lightwave circuit  2 A, respectively. Namely, one ends of the optical waveguides  5   1  to  5   4  and  6   1  to  6   4  are in contact with one end facet (left side end facet in  FIG. 5 ) of the PLC platform  4 , respectively, and the other ends thereof are in contact with the other end facet (right side end facet in  FIG. 5 ) of the PLC platform  4 , respectively. 
     Additionally, spot size converters (not shown) are formed in input/output portions of each of the semiconductor waveguides  10   1  to  10   4  and  11   1  to  11   4  optically coupled with each of the corresponding optical waveguides  5   1  to  5   4  and  6   1  to  6   4 . By the spot size converters, it is able to increase coupling efficiency by matching the spot sizes between each of the semiconductor waveguides  10   1  to  10   4  and  11   1  to  11   4  and each of the optical waveguides  5   1  to  5   4  and  6   1  to  6   4 . 
     Moreover, it is fabricated in order to make an optical gain in a TE mode and an optical gain in a TM mode be same with each other also for an active layer portion of each of the semiconductor optical amplifiers  9   1  to  9   4 . By this construction, polarization independent operation can be achieved also involving the optical waveguides  5   1  to  5   4  and  6   1  to  6   4  and the semiconductor waveguides  10   1  to  10   4  and  11   1  to  11   4 . 
     In the optical integrated circuit  1 A depicted in  FIGS. 5 and 6 , the turnaround portions  11   a  turned on the semiconductor substrate  8  are formed, and a plurality of (four) turnaround waveguides  90  connected to the respective output port of the semiconductor optical amplifier (SOA)  9   1  to  9   4  are arranged. 
     According to the second embodiment having the configurations described above, following functions and effects can be obtained in addition to the functions and effects obtained by the aforementioned first embodiment. 
     Even when the optical integrated circuit  1 A is fabricated by integrating the semiconductor element  3 A in which a plurality of semiconductor optical amplifiers  9   1  to  9   4  are arranged in array pattern, and the planar lightwave circuit  2 A, the optical alignment works can be easily performed and excellent coupling efficiency can also be obtained. 
     Although there are many waveguides joined at the contact surface  12 , the planar lightwave circuit  2 A and the semiconductor element  3 A are made contact with each other at one contact surface  12  to be fixed, so that it is possible to perform the alignment and fixing works of the planar lightwave circuit  2 A and the semiconductor element  3 A at once. 
     Since the output semiconductor waveguides  11   1  to  11   4  formed on the output side of each of the semiconductor optical amplifiers  9   1  to  9   4  have the turnaround portions  11   a,  respectively, the radiuses of curvature of the turnaround portions  11   a  can be reduced, thus allowing a size of the semiconductor element  3 A to be greatly reduced. Hence, the compact optical integrated circuit  1 A in which the planar lightwave circuit  2 A and the semiconductor element  3 A are integrated can be achieved. 
     The spot size converters (not shown) are formed in the input/output portions of each of the semiconductor waveguides  10   1  to  10   4  and  11   1  to  11   4  coupled with each of the corresponding optical waveguides  5   1  to  5   4  and  6   1  to  6   4 . By the spot size converters, the coupling efficiency can be increased by matching the spot sizes between each of the optical waveguides  10   1  to  10   4  and  11   1  to  11   4  and each of the optical waveguides  5   1  to  5   4  and  6   1  to  6   4 . 
     Third Embodiment 
     Next, an optical integrated circuit in accordance with a third embodiment will be described based on  FIG. 7 .  FIG. 7  is a conceptual diagram showing a schematic configuration of an optical integrated circuit  1 B in accordance with the third embodiment. 
     The optical integrated circuit  1 B is characterized by following configurations. 
     A semiconductor element  3 B in which a plurality of waveguide-type photodiodes (elements)  30   1  to  30   6  are formed on the semiconductor substrate  8  in array pattern, and a planar lightwave circuit  2 B in which a plurality of optical waveguides are formed are fixed at one contact surface  12  and are integrated. As an example, six waveguide photodiodes  30   1  to  30   6  are formed on the semiconductor substrate  8  in the present embodiment. 
     Six straight optical waveguides  31   1  to  31   6  respectively coupled with the light incidence side end facets (light receiving facets) of the waveguide photodiodes  30   1  to  30   6  are formed on the PLC platform  4  of the planar lightwave circuit  2 B. 
     Each of the waveguide photodiodes  30   1  to  30   6  is an element with only one input and without an optical output, and each of the waveguide photodiodes  30   1  to  30   4  is coupled with each of the optical waveguides  31   1  to  31   6  only at one side. For that reason, ends of the input semiconductor waveguides inside of the waveguide photodiodes  30   1  to  30   6  are in contact with the end facet  3   a  of the semiconductor element  3 , respectively, and are coupled with the corresponding optical waveguides  31   1  to  31   6  of the planar lightwave circuit  2 B at the contact surface  12 . 
     A turnaround waveguide  32  for alignment for letting a light for alignment pass is formed aside from the waveguide photodiodes  30   1  to  30   6  on the semiconductor substrate  8 . The turnaround waveguide  32  for alignment has bent portions  32   a  at two points, and an end on the light input side and an end on the light output side thereof are in contact with the end facet  3   a  of the semiconductor element  3 B, respectively. 
     A first optical waveguide  33  for alignment and a second optical waveguide  34  for alignment for guiding the light for alignment are formed on the PLC platform  4 . 
     The end on the light input side and the end on the light output side of the turnaround waveguide  32  for alignment are coupled with an end of the first optical waveguide  33  for alignment and an end of the second optical waveguide  34  for alignment at the contact surface  12 , respectively. 
     When the optical integrated circuit  1 B having the configurations described above is fabricated, the active alignment similar to that of the aforementioned first embodiment is performed. In this case, when the light for alignment is entered into the first optical waveguide  33  for alignment, the light will be emitted from the second optical waveguide  34  for alignment passing through the turnaround waveguide  32  for alignment and the second optical waveguide  34  for alignment. The output light is received by a light receiving element (not shown), a relative position between the planar lightwave circuit  2 B and the semiconductor element  3 B is adjusted so that the amount of light received may be the maximum level, and both of them are then fixed at the contact surface  12 . 
     According to the third embodiment having the configurations described above, following functions and effects can be obtained in addition to the functions and effects obtained by the aforementioned first embodiment. The space between waveguides both in the semiconductor substrate  8  and in the PLC platform  4  is formed with very high accuracy. Therefore, by optimizing coupling efficiency between the turnaround waveguides  32  for alignment and the corresponding optical waveguides  33  and  34  for alignment on the PLC platform  4  by the aforementioned active alignment, it allows also the coupling between the waveguide photodiodes  30   1  to  30   6  and the corresponding optical waveguides  31   1  to  31   6  to be simultaneously optimized. 
     Fourth Embodiment 
     Next, an optical integrated circuit module  1 C in accordance with a fourth embodiment will be described based on  FIG. 8 .  FIG. 8  is a conceptual diagram showing a schematic configuration of the optical integrated circuit module  1 C in accordance with the fourth embodiment. 
     The optical integrated circuit module  1 C is characterized by following configurations. 
     In the aforementioned second embodiment shown in  FIG. 5 , RF electrodes  55  for supplying RF signals to the semiconductor optical amplifiers  9   1  to  9   4  are formed on the silicon substrate  7  composing the semiconductor substrate together with the semiconductor substrate  8 . The RF signals can be individually inputted into respective semiconductor optical amplifiers  9   1  to  9   4  from the RF electrodes  55  via wires  54 . 
     A fiber array composed of optical fibers  35   1  to  35   8  for input/output coupled with one ends of the optical waveguides  5   1  to  5   4  and  6   1  to  6   4 , respectively, is connected to an end facet (one end facet  2   a  of the planar lightwave circuit  2 ) opposite to the contact surface  12  of the PLC platform  4 . 
     In the optical integrated circuit  1 C depicted in  FIG. 8 , the turnaround portions  11   a  turned on the semiconductor substrate  8  are formed, and a plurality of (four) turnaround waveguides  90  connected to the respective output port of the semiconductor optical amplifier (SOA)  9   1  to  9   4  are arranged. 
     According to the fourth embodiment having the configurations described above, following functions and effects can be obtained in addition to the functions and effects obtained by the aforementioned second embodiment. The propagating direction of light is turned around at the turnaround portion  11   a  of each of the output semiconductor waveguides  11   1  to  11   4  formed on the semiconductor substrate  8 , so that connection between the planar lightwave circuit  2 A and the optical fibers  35   1  to  35   8  can also be made only by one end facet (one end facet  2   a  of the planar lightwave circuit  2 A), thus allowing also the alignment and joint works between the planar lightwave circuit  2 C and the optical fibers  35   1  to  35   8  to be performed at once. 
     Since joint of the waveguides between the planar lightwave circuit  2 C and the semiconductor element  3 C can be made only at the end facet  3   a  of the semiconductor element  3 C (end facet of one side of the silicon substrate  7 ), an electrode setting space can be formed on a side where the semiconductor element  3 C is not fixed with the planar lightwave circuit  2 A to thereby provide the RF electrodes  55  in this space. As a result of this, it is very effective in driving the semiconductor optical amplifiers  9   1  to  9   4  at high speed. 
     Fifth Embodiment 
     Next, an optical integrated circuit module  1 D in accordance with a fifth embodiment will be described based on  FIG. 9 .  FIG. 9  is a conceptual diagram showing a schematic configuration of the optical integrated circuit module  1 D in accordance with the fifth embodiment. 
     The optical integrated circuit module  1 D is characterized by following configurations. 
     A planar lightwave circuit  2 D in which an arrayed waveguide grating (AWG)  40  is formed on the PLC platform  4 , and a semiconductor element  3 D in which a plurality of light receiving elements  50   1  to  50   n  of semiconductor waveguide type (N pieces) are formed on the semiconductor substrate  8  are fixed at one contact surface  12  and are integrated. The waveguide photodiodes  50   1  to  50   n  have a configuration similar to that of the aforementioned waveguide photodiodes  30   1  to  30   6  shown in  FIG. 7 . 
     The arrayed waveguide grating (AWG)  40  is composed of one input waveguide  41 , a group of n output waveguides  42   1  to  42   n , an input side slab waveguide  43 , an output side slab waveguide  44 , and an arrayed waveguide  45 . 
     Each of the waveguide photodiodes  50   1  to  50   n  is an element with only one input and without an optical output, and each of the waveguide photodiodes  50   1  to  50   n  is coupled with each of the optical waveguides  42   1  to  42   n  only at one side. For that reason, ends inside the waveguide photodiodes  50   1  to  50   n  are in contact with the end facet  3   a  of the semiconductor element  3 D, respectively, and are coupled with the group of the corresponding output waveguides  42   1  to  42   n  of the planar lightwave circuit  2 D at the contact surface  12 . 
     A turnaround waveguide  51  for alignment for letting a light for alignment pass is formed aside from the waveguide photodiodes  50   1  to  50   n  on the semiconductor substrate  8 . The turnaround waveguide  51  for alignment has bent portions  51   a  at two points, and an end on the light input side and an end on the light output side thereof are in contact with the end facet  3   a  of the semiconductor element  3 D, respectively. 
     A first optical waveguide  46  for alignment and a second optical waveguide  47  for alignment for guiding the light for alignment are formed on the PLC platform  4 . 
     The end on the light input side and the end on the light output side of the turnaround waveguide  51  for alignment are coupled with an end of the first optical waveguide  46  for alignment and an end of the second optical waveguide  47  for alignment at the contact surface  12 , respectively. 
     Optical fibers  61 ,  62 , and  63  are connected to an end of the input waveguide  41  of the arrayed waveguide grating (AWG)  40 , the other end of the first optical waveguide  46  for alignment, and the other end of the second optical waveguide  47  for alignment, respectively. 
     In the optical integrated circuit module  1 D, the arrayed waveguide grating  40  is used as a splitter. When the optical integrated circuit module  1 D having the configuration described above is fabricated, the active alignment similar to that of the aforementioned third embodiment shown in  FIG. 7  is performed. In this case, when a light for alignment is entered into the first optical waveguide  46  for alignment from the optical fiber  62 , the light will be emitted from the optical fiber  63  passing through the turnaround waveguide  51  for alignment and the second optical waveguide  47  for alignment. The output light is received by a light receiving element (not shown), a relative position between the planar lightwave circuit  2 D and the semiconductor element  3 D is adjusted so that the amount of light received may be the maximum level, and both of them are fixed at the contact surface  12 . 
     According to the fifth embodiment having the configurations described above, following functions and effects can be obtained in addition to the functions and effects obtained by the aforementioned first embodiment. The space between waveguides both in the semiconductor substrate  8  and in the PLC platform  4  is formed with very high accuracy. For this reason, by optimizing coupling efficiency between the turnaround waveguides  51  for alignment and the corresponding optical waveguides  46  and  47  for alignment on the PLC platform  4  by the aforementioned active alignment, it allows also the coupling between the waveguide photodiodes  50   1  to  50   n  and the output waveguide groups  42   1  to  42   n  of the corresponding arrayed waveguide grating  40  to be simultaneously optimized. 
     Sixth Embodiment 
     Next, an optical integrated circuit  1 E in accordance with a sixth embodiment will be described based on  FIG. 10 . 
     The feature of the optical integrated circuit lies in that a spot size converter is arranged in a portion of coupling the optical waveguide with the semiconductor waveguide to match the spot sizes between the optical waveguides  5 ,  6  and the corresponding semiconductor waveguides  10 ,  11  in the optical integrated circuit  1  according to the first embodiment depicted in  FIG. 2 . As depicted in  FIG. 10 , the spot size converters  71 ,  72  are arranged to the semiconductor element  3 E side as one example in this embodiment. The spot size converter  71  comprises a tapered waveguide (a wide width flared type spot size converter) with the width (the width of the core in the plane of the paper in  FIG. 10 ) of the waveguide varied to be a tapered shape so as to match the spot size (S 1 ) of the optical waveguide  5  in the planer lightwave circuit  2 E with the spot size (S 2 : S 1 &gt;S 2 ) of the input side semiconductor waveguide  10 . At the same time, the spot size converter  72  comprises a tapered waveguide with the width of the waveguide varied to be a tapered shape so as to match the spot size (S 1 ) of the optical waveguide  6  with the spot size (S 2 ) of the output side semiconductor waveguide  11 . 
     Furthermore, in the optical integrated circuit  1 E depicted in  FIG. 10 , the turnaround portions  11   a  turned on the semiconductor substrate  8  are formed, and two semiconductor optical amplifiers (SOA)  9   1 ,  9   2  as the element are arranged in a single turnaround waveguide  90  respectively connected to the input port and the output port of the element. 
     In addition, although the spot size converters  71 ,  72  are arranged to the semiconductor element  3 E side in  FIG. 10 , the spot size converter may be arranged to the planer lightwave circuit  2 E side, or both sides of the semiconductor element  3 E and the planer lightwave circuit  2 E. The spot size converter is not limited to the wide width flared type spot size converter, and the spot size converter with other structures may be applicable. More specifically, the spot size converter may be the waveguide including an end portion having a wide width of the waveguide corresponding to the spot size (S 1 ) of the optical waveguide  5 ,  6  and the other end portion having a narrow width of the waveguide corresponding to the spot size (S 2 ) of the semiconductor waveguide  10 ,  11 . 
     (According to the Sixth Embodiment Having the Above Configuration, Following Functions and Effects can be Obtained.) 
     According to the spot size converter  71 ,  72 , the spot sizes between the optical waveguide  5  and the semiconductor waveguide  10  as well as the spot sizes between the optical waveguide  6  and the semiconductor waveguide  11  can be respectively matched at the coupling portion. More specifically, the spot sizes between the output port of the optical waveguide  5  and the input port of the turnaround waveguide  90 , as well as the spot sizes between the input port of the optical waveguide and the output port of the turnaround waveguide can be respectively matched. Thus, it becomes possible to reduce coupling loss at the coupling portion so as to obtain high coupling efficiency. 
     Seventh Embodiment 
     Next, an optical integrated circuit  1 F in accordance with a Seventh embodiment will be described based on  FIG. 11 . 
     The feature of the optical integrated circuit depicted in  FIG. 11  lies in that the structure in which the beam obliquely enters into or emits from on each of the end facet is applied so as to reduce reflection on each of the end facet of the optical waveguide  5 ,  6  or each of the end facet of the semiconductor waveguide in the optical integrated circuit according to the aforementioned first embodiment. 
     As depicted in  FIG. 11 , an inclined waveguide  73 ,  76  with an center axis of the core C 1 , C 2  inclined against the end facet  2   b  is arranged on each of the end portion of the end facet  2   b  side in the optical waveguide  5 ,  6  so as to cause the beam to obliquely enter into or emit from each of the end facet  5 ,  6  in the optical waveguide  5 ,  6 . The inclined waveguide  73  of the optical waveguide  5  is connected to the waveguide  75  having the center axis C 3  of the core perpendicular to each of the end facet  2   a,    2   b  through a bent waveguide  74 . The optical waveguide  5  comprises those waveguides  73  to  75 . The inclined waveguide  76  of the optical waveguide  6  is connected to the waveguide  78  having the center axis C 4  of the core perpendicular to each of the end facet  2   a,    2   b  through a bent waveguide  77 , in the same manner as the above. The optical waveguide  6  comprises those waveguides  76  to  78 . 
     On the other hand, an inclined waveguide  81 ,  82  with an center axis of the core inclined against the end facet  3   b  is arranged on each of the end portion of the end facet  3   a  side in the semiconductor waveguide  10 ,  11  so as to cause the beam to obliquely enter into or emit from each of the end facet in the semiconductor waveguide  10 ,  11 . The inclined angle θ c 5  of the center axis C 5  of the core in the inclined waveguide  81  may be determined by the effective refractive index n 73  of the inclined waveguide  73  and the inclined angle θ c 1  of the center axis C 1  of the core, as well as the effective refractive index n 81  of the inclined waveguide  81  so as to satisfy the following Snell&#39;s law: 
         n   81  sin θ c 5= n   73  sin θ c 1 
     The inclined angle of the center axis C 6  of the core of the inclined waveguide  82  may be determined in the same manner. The aforementioned inclined waveguide  81 ,  82  is the same spot size converter as the spot size converter  71 ,  72  depicted in  FIG. 10 . The inclined waveguide  81  is connected to the input semiconductor waveguide  10  through the bent waveguide  83 , while the inclined waveguide  82  is connected to the output semiconductor waveguide  11  through the bent waveguide  84 . 
     (According to the Seventh Embodiment Having the Above Configuration, Following Functions and Effects can be Obtained.) 
     In the planer lightwave circuit  2 F side, the beam emitted from or entered into each end facet of the inclined waveguide  73 ,  76  does not emit from or enter into in the direction perpendicular to each end facet, but emits from or enters into in the direction inclined. According to the above, the reflected light at each end facet of the inclined waveguide  73 ,  76  becomes difficult to be coupled again with the inclined waveguide  73 ,  76 , thus enabling to reduce equivalent reflectance at each end facet of the inclined waveguide  73 ,  76 . 
     In the same manner, in the planer lightwave circuit  3 F side, the beam emitted from or entered into each end facet of the inclined waveguide  81 ,  82  (entering end facet or emitting end facet of the semiconductor waveguide  10 ,  11 ) does not emit from or enter into in the direction perpendicular to each end facet, but emits from or enters into in the direction inclined. According to the above, the reflected light at each end facet of the inclined waveguide  81 ,  82  becomes difficult to be coupled again with the inclined waveguide  81 ,  82 , thus enabling to reduce equivalent reflectance at each end facet of the inclined waveguide  81 ,  82 . 
     In the optical integrated circuits described in each of the aforementioned embodiments, the reflection at the coupling portion between the planer lightwave circuit  2 F and the semiconductor element  3 F likely causes deterioration in the characteristics because of applications. However, according to the optical lightwave circuit  1 F of the embodiment, those deterioration in the characteristics can be reduced. 
     When the structure in which the beam obliquely enters into or emits from at each end facet of the optical waveguide  5 ,  6  and the semiconductor waveguide  10 ,  11  and non-reflecting coating or a window-facet structure are combined in use, the reflection at each end facet of the optical waveguide  5 ,  6  and the semiconductor waveguide  10 ,  11  can be further effectively reduced. In case that non-reflecting coating is applied on each end facet of the inclined waveguide  73 ,  76  and the inclined waveguide  81 ,  82  it is important to design the coating film considering the refractive index of the material in a gap portion (for example, refractive index of a UV adhesive agent or air) between the planer lightwave circuit  2 F and the semiconductor element  3 F. 
     Eighth Embodiment 
     Next, an optical integrated circuit  1 G in accordance with an Eighth embodiment will be described based on  FIG. 12 . 
     The feature of the optical integrated circuit  1 G lies in that the optical integrated circuit has a structure in which one planer lightwave circuit  2 G and two semiconductor elements  3 G 1 ,  3 G 2  are fixed at one contact surface. The number of the semiconductor elements is not limited to two, and at least three semiconductor elements can be applicable. Furthermore, the present invention is applicable to the optical integrated circuit having a structure in which a plurality of optical integrated circuits and a plurality of semiconductor elements are connected at one contact surface. 
     As depicted in  FIG. 12 , an optical waveguide  5 A, an arrayed waveguide grating (AWG) 40 A, a plurality of input waveguide (optical waveguide)  85   1  to  85   8  connected to the input side slab waveguide  43  A of the AWG  40 A and an output waveguide (optical waveguide)  6 A connected to the output side slab waveguide  44 A of the AWG  40 A are formed in the planer lightwave circuit  2 G. 
     The output waveguide  6 A is an inclined waveguide  6 A′ with the center axis thereof inclined to the end facet  2   b  in the same manner as the inclined waveguide  73 ,  76  depicted in  FIG. 11 . The end portion of the end facet  2   b  side in the optical waveguide  5 A is an inclined waveguide  5 A′ in the same manner as the inclined waveguide  73 ,  76 . Furthermore, each input port of the input waveguide  85   1  t  85   8 , an output port of the output waveguide  6 A, and an input port of the waveguide  5  are on the end facet  2   b.    
     The semiconductor  3 G 1  has substantially the same configuration as the semiconductor  3 F depicted in  FIG. 11 . However, the inclined waveguide  10   a,    11   b  with the center axis thereof inclined to the end facet  3   a  is arranged on each end portion of the end facet  3   a  side of the semiconductor waveguide  10 ,  11  in the semiconductor  3 G 1 . 
     In addition, the semiconductor  3 G 2  includes the semiconductor substrate  8   a  and a plurality of semiconductor light emitting elements (Element) such as semiconductor laser diode or the like formed on the substrate  8   a.  In the embodiment, eight semiconductor light emitting elements  80   1  to  80   8  emitting lights having different wavelengths are formed on the semiconductor substrate  8   a  for example. According to the semiconductor light emitting elements  80   1  to  80   8 , a multiple wavelengths semiconductor laser element can be configured. 
     The planer lightwave circuit  2 G and the semiconductor element  3 G 1  are fixed to be integrated at a single contact surface  12   1 , while the planer lightwave circuit  2 G and the semiconductor element  3 G 2  are fixed to be integrated at a single contact surface  12   2  in the optical integrated circuit  1 G. 
     The inclined waveguide  5 A′,  6 A′ of the planer lightwave circuit  2 G and the inclined waveguide  10   a,    11   b  of the semiconductor element  3 G 1  are optically coupled, respectively on the contact surface  12   1 . In addition, the input waveguides  85   1  to  85   8  of the planer lightwave circuit  2 G and respect optical output ports of the semiconductor light emitting elements  80   1  to  80   8  are optically coupled on the contact surface  12   2 . 
     (According to the Eighth Embodiments Having the Above Configuration, Following Functions and Effects can be Obtained.) 
     One planer lightwave circuit  2 G and a plurality of semiconductor elements (two semiconductor elements  3 G 1 ,  3 G 2 ) having different functions are integrated so as to enable to realize more complicated functions. 
     It enables an optical integrated circuit in which an optical signal is transmitted from the semiconductor element (A) to the planer lightwave circuit (PLC) to the semiconductor element (B) which has different function from the semiconductor element (A) further to the planer lightwave circuit (PLC). More specifically, it enables a compact integration even though optical active components such as a semiconductor light receiving element and optical passive components are combined to be mounted. 
     Ninth Embodiment 
     Next, an optical integrated circuit  1 H in accordance with a Ninth embodiment will be described based on  FIG. 13 . 
     The feature of the optical integrated circuit  1 H lies in the structure in which a leaner type straight semiconductor waveguide  91  is arranged to the semiconductor element  3 H side in addition to at least one turnaround waveguide  90 , and an optical waveguide  87  coupled to a straight semiconductor waveguide  91  is arranged to the planer lightwave circuit  2 H side in addition to an optical waveguide  5 ,  6  in the optical integrated circuit  1 E depicted in  FIG. 10 . The output port of the straight semiconductor waveguide  91  is positioned on the end facet  3   a,  the input port of which is connected to an optical output port of the semiconductor light emitting element  80  (end portion of the semiconductor waveguide). 
     According to the optical integrated circuit  1 H, it is possible to realize a compact optical integrated circuit having a complex function due to the fact that the elements having different functions are mounted on a single semiconductor element  3 . 
     Tenth Embodiment 
     Next, an optical integrated circuit  1 J in accordance with a Tenth embodiment will be described based on  FIG. 14 ,  15 .  FIG. 14  is a sectional view along C-C line in  FIG. 10 .  FIG. 15  is a sectional view along D-D line in  FIG. 10 . 
     The feature of the optical integrated circuit  1 J lies in that the turnaround waveguide  90  is configured to be a high mesa structure having a low loss even if the radius of curvature is small, and each semiconductor waveguide in the inside of the semiconductor optical amplifier (SOA)  9   1 ,  9   2  is configured to be an buried mesa structure in the optical integrated circuit  1 E depicted in  FIG. 10 . 
     Each of the semiconductor waveguide in the inside of the semiconductor optical amplifier (SOA)  9   1 ,  9   2  includes a lower cladding layer  20  formed on the semiconductor substrate  8 , an active layer (core layer)  23  of optical amplifying medium formed on the lower cladding layer  20 , and an upper cladding layer formed on the active layer  23 . Each of the semiconductor wavegude in the inside of the semiconductor optical amplifier (SOA)  9   1 ,  9   2  is embedded by the first current block layer  93 , the second current block layer  94  formed on the first current block layer  93 , and the cladding layer  95  formed on the second current block layer  94 . 
     On the other hand, as depicted in  FIG. 15 , the turnaround portion  11   a  of the turnaround waveguide  90  is formed so as to be a high mesa structure including the lower cladding layer  20 , the core layer  21 , and the upper cladding layer  22 , in the same manner as the optical integrated circuit  1  depicted in  FIGS. 2 and 4 . According to the fact that each of the semiconductor waveguide is configured to be a high mesa structure, the difference in the refractive index between the core layer  21  and the air in the both sides thereof is so remarkably large as for example at least 40%, thus it is possible to maintain low loss even if the radius of curvature of the turnaround portion becomes small. 
     The respective spot size converters  71 ,  72  (refer to  FIG. 10 ) arranged on each end portion of the input side of the semiconductor waveguide  10 , output side of the semiconductor waveguide, and the semiconductor waveguide  10 ,  11  are formed so as to be mesa structure in the same manner as the turnaround portion  11   a.  It is possible that the respective spot size converters  71 ,  72  are formed so as to be an embedded mesa type high mesa structure in the same manner as each of the semiconductor waveguide in the inside of the semiconductor optical amplifier (SOA)  9   1 ,  9   2 . 
     Method for Mounting 
     A method for mounting the semiconductor element in the aforementioned each of the optical integrated circuit is described hereunder. 
     At first, the first method (silicon bench type) for mounting the semiconductor element  3  on the silicon substrate (silicon bench)  7  is described with reference to  FIGS. 16 and 17 .  FIGS. 16 and 17  show the optical integrated circuit, in order to simplify the description of the method for mounting the semiconductor element  3  in the optical integrated circuit described in the aforementioned embodiments, in which the planer lightwave circuit  2  has one optical waveguide  100  and the semiconductor element  3  has one semiconductor waveguide  200 . The semiconductor substrate  8  of the semiconductor element  3  is not shown in  FIG. 16 . 
     The first method for mounting includes the following steps:
         (1) at first, the semiconductor element  3  is mounted on the silicon substrate (the third substrate)  7  as the silicon bench;   (2) then, active alignment is performed between the optical waveguide  100  of the planer lightwave circuit  2  and the semiconductor waveguide  200  of the semiconductor element  3  so as to optimally optically coupled; and   (3) then, the planer lightwave circuit  2 , the semiconductor element  3  and the silicon substrate  7  are fixed by an adhesive agent  300 .       

     Thus, the optical integrated circuit is fabricated in which the planer lightwave circuit  2  is connected to the semiconductor element  3  on a single contact surface  12  (refer to  FIG. 1 ), and at the same time the planer waveguide  100  is optically coupled and fixed with the semiconductor waveguide  200 . 
     According to the aforementioned method for mounting, the semiconductor  3  is mounted on the silicon substrate  7 , and the active alignment is performed, thus obtaining optimum low coupling loss. 
     Then, the second method (silicon terrace type) for mounting the semiconductor element  3  on the terrace (silicon terrace  400 ) formed in the planer lightwave circuit substrate (PLC) (for example, silicon substrate)  4  is described with reference to  FIGS. 18 and 19 . The semiconductor substrate  8  of the semiconductor element  3  is not shown in  FIG. 18 . 
     The second method for mounting includes the following steps:
         (1) at first, the planer lightwave circuit  2  is prepared on the planer lightwave circuit substrate (for example, silicon substrate)  4  with a silicon terrace provided on which the semiconductor element is to be mounted;       

     A terrace working is implemented in advance in a portion on which the semiconductor element is mounted in the silicon terrace  400 . As for the terrace working, for example, a circuit pattern is formed on the silicon terrace, then a pad is formed on the circuit pattern, and then a solder layer is formed on the pad.
         (2) then, the semiconductor  3  is mounted with high precision by bonding on the silicon terrace  400 .       

     According to the above process, Au, Cu or solder bump provided in one electrode of the semiconductor element  3  and the pad on the silicon terrace  400  are jointed by solder so that the semiconductor element  3  is fixed to the planer lightwave circuit (PLC) substrate. The numeral reference  500  represents solder joint portion in  FIGS. 18 and 19 . 
     Thus, the optical integrated circuit is fabricated in which the planer lightwave circuit  2  is connected to the semiconductor element  3  on a single contact surface  12  (refer to  FIG. 1 ), and at the same time the planer waveguide  100  is optically coupled and fixed with the semiconductor waveguide  200 . 
     An index-matching oil may be filled between the planer lightwave circuit  2  and the semiconductor element  3  so as to reduce reflection on the end facet. Alternatively, an adhesive agent  300  may be applied between the planer lightwave circuit  2  and the semiconductor element  3  so as to enhance the fixing. 
     According to the aforementioned method for mounting, since the semiconductor element  3  is mounted on the silicon terrace  400  formed in the planer lightwave circuit (PLC) substrate  4  of the planer lightwave circuit, it is possible to exclude the silicon substrate  7  for mounting the semiconductor element  3  described in the aforementioned each embodiment. 
     Fixing by Adhesive Agent 
     It is preferable in the step (3) in the aforementioned first method for mounting that the active alignment is performed so as to optimally coupled the optical waveguide  100  of the planer lightwave circuit  2  with the semiconductor waveguide  200  of the semiconductor waveguide  200 , and then, under the above condition, the planer lightwave circuit  2 , the semiconductor element  3  and the silicon substrate  7  are fixed by such adhesive agent as a UV hardening adhesive agent, a thermal hardening adhesive agent or the like. It is possible to maintain highly reliable high precision alignment of the waveguide for a long period of time by fixing the planer lightwave circuit  2 , the semiconductor element  3  and the silicon substrate  7  with the use of such adhesive agent as a UV hardening adhesive agent, a thermal hardening adhesive agent or the like. 
     Incidentally, the planer lightwave circuit  2  and the semiconductor element  3  may be respectively fixed in combination with such adhesive agent as a UV hardening adhesive agent, a thermal hardening adhesive agent or the like so as to enhance the fixing. 
     Method for Optical Alignment 
     Then, a method for optical alignment in the aforementioned optical integrated circuit is described. 
     At first, the first method for optical alignment (ASE alignment) in which an active alignment is performed with the use of light emitted from the semiconductor element is described. 
     The first method for optical alignment is applied to the optical integrated circuit including a light source emitting such as an ADE (Amplified Spontaneous Emission) light or laser light as one of a plurality of elements arranged in array pattern on the semiconductor substrate  8  of the semiconductor element. The first method for optical alignment is described with the use of the optical integrated circuit configured with an ASE light source arranged in place of one (for example, light receiving element  30   1 ) of a plurality of semiconductor waveguide type light receiving element  30   1  to  30   6  in the optical integrated circuit  1 B depicted in  FIG. 7 , for example. 
     In the first method for optical alignment, the ASE light emitted from the ASE light source is entered into one of the optical waveguide coupled with the ASE light source of a plurality of optical waveguides  311  to  316  under the condition in which the planer lightwave circuit  2 B is placed in close to the semiconductor element  3 B, then the light emitted from the output port through the optical waveguide is received by the light receiving element, and then an active alignment is performed between the planer lightwave circuit  2  and the semiconductor element  3  in such manner that the amount of the receiving light becomes maximum. 
     According to the above method for optical alignment, since the active alignment is performed with the use of the SAE light from the ASE light source arranged in the semiconductor element, it is not necessary to arrange the turnaround waveguide for alignment in the optical integrated circuit in which a plurality of elements are arranged in array pattern on the semiconductor substrate  8  of the semiconductor element such as the optical integrated circuit  1 B depicted in  FIG. 7 . 
     Then, the second method for optical alignment (amplifying alignment) performing an active alignment by introducing light from outside is described. 
     The second method for optical alignment is applied to the optical integrated circuit in which one or a plurality of semiconductor optical amplifier is arranged on the semiconductor substrate  8  of the semiconductor element. 
     The second method for optical alignment is described with the use of the optical integrated circuit  1  depicted in  FIG. 1 . 
     In the second method for optical alignment, the active alignment is performed by introducing light from outside under the condition in which the planer lightwave circuit  2  is placed in close to the semiconductor element  3  and the current is applied to the semiconductor optical amplifier  9 . 
     More specifically, the light for alignment is entered from the incident port  5   a  side the optical waveguide  5  to the optical waveguide  5 , then the light emitted from an emitting port  6   a  through the semiconductor waveguide  10 , semiconductor optical amplifier  9 , semiconductor waveguide  11  and the optical waveguide  6  is received by the light receiving element, and then the active alignment between the planer lightwave circuit  2  and the semiconductor element  3  is performed in such manner that the amount of the received light becomes maximum. 
     According to the aforementioned method for optical alignment, since the light introduced from outside (light for alignment) is amplified by the semiconductor optical amplifier  9  and entered into the light receiving element, light receiving sensitivity becomes high so as to enable high precision active alignment. Furthermore, when the aforementioned method for optical alignment using the light introduced from outside is applied, the active alignment can be performed even if the semiconductor element does not include the light emitting element, although the improvement in the light receiving sensitivity is not expected. 
     Incidentally, the second method for optical alignment is applicable not only to the optical integrated circuit  1  depicted in  FIGS. 1 and 2 , but also to the optical integrated circuit  1 A depicted in  FIGS. 5 and 6 , the optical integrated circuit  1 E depicted in  FIG. 10 , the optical integrated circuit  1 F depicted in  FIG. 11 , the optical integrated circuit  1 G depicted in  FIG. 12 , the optical integrated circuit  1 H depicted in  FIG. 13 , and the optical integrated circuit  1 J depicted in  FIGS. 14 and 15 . 
     Reverse Connection of the PLC (Planer Lightwave Circuit) 
     It is preferable in the aforementioned each optical integrated circuit that the PLC (planer lightwave circuit) chip (for example, the planer lightwave circuit  2 ) is connected to the silicon substrate (silicon bench)  7  in reverse direction, i.e., the reverse connecting configuration in which the PLC chip and the silicon substrate are placed upside down to be connected. One example of the optical integrated circuit having the reverse connecting configuration is described with reference to  FIG. 20 .  FIG. 20  shows the optical integrated circuit, in order to simplify the description of the optical integrated circuit, in which the planer lightwave circuit  2  has one optical waveguide  100  and the semiconductor element  3  has one semiconductor waveguide  200 . The semiconductor substrate  8  of the semiconductor element  3  is not shown in  FIG. 20 . 
     As described in the aforementioned each embodiment, the semiconductor element (semiconductor waveguide chip)  3  is mounted on the silicon substrate (silicon bench)  7 . The optical waveguide of SiO2 is formed on the PLC substrate  4  in the planer lightwave circuit (PLC chip)  2 . In general, when the direction of the semiconductor element  3  mounted on the silicon substrate  7  is referred to the upper side, the optical waveguide is placed in the upper side to the PLC substrate  4 , and both of the silicon bench  7  and the PLC substrate  4  are together positioned in the lower side. This is called as a normal connection, since the relation between the optical waveguide and the substrate is the same. 
     All the optical integrated circuits described in the aforementioned each embodiment have the normal connecting configuration. In the normal connecting configuration, it is difficult to sufficiently harden the UV hardening adhesive agent  300  filled between the planer lightwave circuit  2  and the semiconductor element  3  as well as between the both substrates  4 ,  7 , since the UV light does not transmit the silicon (Si), and the thickness of each substrate  4 ,  7  is as thick as of about  1  mm while the gap between the planer lightwave circuit  2  and the semiconductor element  3  is as thin as of about a few urn. 
     Thus, it is preferable to have the reverse configuration in which the planer lightwave circuit  2  is turned upside down to the silicon substrate  7  in the aforementioned each embodiment, as depicted in  FIG. 20 . 
     The optical integrated circuit depicted in  FIG. 20  has the reverse configuration in which an upper plate  600  made of glass fixed in advance by the UV hardening adhesive agent is mounted on the planer lightwave circuit  2  (on the face opposite to the silicon substrate  7 ), and then the planer lightwave circuit  2  is turned upside down to the silicon substrate  7 . More specifically, the planer lightwave circuit  2  is turned upside down to the silicon substrate  7 , and the end face of the upper plate  600  and the silicon substrate  7  is jointed by the adhesive agent  300 . 
     According to the optical integrated circuit depicted in  FIG. 20 , since the end face of the upper plate  600  on the planer lightwave circuit  2  becomes the adhering face with the silicon substrate  7 , the UV light transmits the upper plate made of glass so as to enable to sufficiently harden the UV hardening adhesive agent  300  placed in the gap. 
     Incidentally, the present invention can also be embodied by being changed as follows. 
     Although the turnaround portion  11   a  is formed in the output semiconductor waveguide  11  in the aforementioned first embodiment shown in  FIG. 1 , the present invention is applicable also to a configuration in which the turnaround portion is formed in the semiconductor waveguide  10  on the input side. Similar effects may be obtained also by this configuration. 
     The present invention is applicable also to an optical integrated circuit module in which, in the aforementioned first, second, third, sixth, seventh, eighth, and ninth embodiments shown in  FIG. 1 ,  FIG. 5 ,  FIG. 7 ,  FIG. 10 ,  FIG. 11 ,  FIG. 12 , and  FIG. 13 , the input/output optical fiber is connected to each of the optical waveguides of the planar lightwave circuit. 
     The present invention is applicable also to an optical integrated circuit in which, in the aforementioned third and fifth embodiments shown in  FIG. 7  and  FIG. 9 , the semiconductor light emitting elements (elements), such as a plurality of semiconductor laser diodes are formed in array pattern on the semiconductor substrate, instead of the plurality of waveguide photodiodes  30   1  to  30   6  and  50   1  to  50   n . When the semiconductor light emitting elements are used as the elements, it is also possible to perform the active alignment while making those semiconductor light emitting elements emit light. In this case, it is not necessary to form the turnaround waveguide  32  for alignment on the semiconductor substrate  8 , and it is not necessary to form the optical waveguides  33  and  34  for alignment also on the PLC platform  4 , either. 
     Although the arrayed waveguide grating (AWG)  40  is used as a splitter in the aforementioned fifth embodiment shown in  FIG. 9 , the present invention is applicable also to an optical integrated circuit or an optical integrated circuit module in which the arrayed waveguide grating  40  is used as an optical multiplexer. The present invention is applicable also to an optical integrated circuit or an optical integrated circuit module, in which a semiconductor element in which a plurality of semiconductor light emitting elements and a plurality of Electro Absorption (EA) modulators utilizing the electric field absorption effect of the semiconductor are arranged in array pattern on the semiconductor substrate of the semiconductor element, and a planar lightwave circuit are fixed at one contact surface, for example.