Patent Publication Number: US-2004057653-A1

Title: Integrated optical element, integrated optical element fabrication method, and light source module

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an integrated optical element in which an optical semiconductor element, such as a semiconductor laser element or a semiconductor optical amplifier, and an optical waveguide in which light output from the optical semiconductor element propagates are integrated, and relates to an integrated optical element fabrication method, and a light source module including the integrated optical element.  
       [0003] 2. Related Background Art  
       [0004] Conventionally, integrated optical elements, in which an optical semiconductor element that is an optical element for generating or amplifying light of a predetermined wavelength, and an optical waveguide in which light output from the optical semiconductor element propagates are integrated, as in the case of a semiconductor laser element (LD: Laser Diode) and a semiconductor optical amplifier (SOA: Semiconductor Optical Amplifier), are known. Examples of this kind of integrated optical element include the integrated optical elements disclosed by Japanese Patent Application Lain-Open Nos. H11-97784 and H11-211924, for example.  
       [0005] Japanese Patent Application Laid-Open No. H11-97784 discloses an external resonator-type frequency stabilized laser comprising a semiconductor laser element and an optical waveguide formed having an optically induced grating. Further, Japanese Patent Application Lain-Open No. H11-211924 discloses an optical circuit in which a silica-based optical waveguide, silica-based optical coupler, and a plurality of semiconductor laser chips of different oscillation wavelengths are integrated.  
       SUMMARY OF THE INVENTION  
       [0006] The present inventors discovered the following problems as a result of investigating the above conventional technologies. That is, in the case of all of the above-described conventional integrated optical elements, the optical semiconductor element and optical waveguide are built on the same silicon (Si) substrate. More specifically, an optical circuit includes a planar waveguide-type optical waveguide is formed on a silicon substrate and an optical semiconductor element chip such as a semiconductor laser element chip is mounted with part of the surface of the silicon substrate on which the optical waveguide is formed.  
       [0007] In this constitution, from the standpoint of the heat dissipation and so forth of the optical semiconductor element, a silicon substrate is preferable as the substrate for mounting an optical semiconductor element such as a semiconductor laser element for outputting light. Further, this silicon substrate makes it possible to accurately form the V grooves and so forth for mounting the optical fiber. However, when an optical waveguide is formed on a silicon substrate, there is the problem that polarization dependence caused by stress-induced birefringence is great, and it is therefore difficult to obtain a favorable optical waveguide.  
       [0008] This invention was conceived in order to resolve the above-mentioned problems, and an object is to provide an integrated optical element in which an optical waveguide having favorable characteristics such as polarization dependence is integrated with an optical semiconductor element, an integrated optical element fabrication method, and a light source module.  
       [0009] In order to achieve the above object, the integrated optical element according to the present invention comprises an optical semiconductor element, an optical circuit element, and a silicon bench having an element mount surface on which the optical semiconductor element and optical circuit element are fixed via a bonding material. The optical semiconductor element includes a light emission layer and outputs light of a predetermined wavelength. The optical circuit element includes a substrate, an optical waveguide provided in correspondence with the optical semiconductor element on the substrate, and a grating formed in the optical waveguide and constituting an external resonator together with the associated optical semiconductor element. Further, the optical semiconductor element is mounted in a flip chip state such the light emission layer is located next to the element mount surface.  
       [0010] Furthermore, the integrated optical element fabrication method according to the present invention involves preparing the above-mentioned silicon bench, preparing the optical semiconductor element, preparing the optical circuit element, and sequentially fixing the optical semiconductor element and optical circuit element on the element mount surface of the silicon bench via the bonding material.  
       [0011] Further, in accordance with the integrated optical element and fabrication method thereof according to the present invention, the optical semiconductor element, which is a semiconductor laser element or a semiconductor optical amplifier, and the optical circuit element, includes an optical waveguide corresponding to this optical semiconductor element, are prepared separately. Further, the integrated optical element is constituted by mounting this optical semiconductor element and optical circuit element on a predetermined surface of the silicon bench that is a substrate prepared separately from the substrate included in the optical circuit element.  
       [0012] As a result of this constitution, the substrates of suitable materials can be used as the substrate on which the optical semiconductor element is mounted and the substrate on which the optical waveguide is formed. Therefore, an integrated optical element having favorable characteristics such as polarization dependence, in which an optical waveguide is integrated with an optical semiconductor element, and a fabrication method therefor realizing favorable characteristics such as polarization dependence can be obtained. Furthermore, because optical devices of two types are fabricated separately, the fabrication yield of the integrated optical element can be improved.  
       [0013] Further, in the case of the integrated optical element according to the present invention, the light emission layer of the optical semiconductor element is shifted further toward the outer periphery side of the cross-section than the center of the cross-section of the optical semiconductor element that is orthogonal to the light emission layer, and the optical waveguide of the optical circuit element is also shifted further toward the outer boundary of the cross-section than the center of the cross-section of the optical circuit element that is orthogonal to the optical waveguide. Here, all of the elements are preferably arranged on the element mount surface of the silicon bench such that the distance between the silicon bench, and the light emission layer and optical waveguide is minimized. In other words, the optical semiconductor element and the optical circuit element are mounted in a flip chip state such that the light emission layer and the optical waveguide are respectively located next to the element mount surface of the silicon bench. As a result, the alignment accuracy between the optical axis of the optical semiconductor element and the optical axis of the optical waveguide of the optical circuit element can be improved.  
       [0014] Further, the optical semiconductor element preferably includes a semiconductor optical amplifier whose end face that faces the optical waveguide is AR (Anti-Reflection) coated. As described above, a grating constituting an external resonator for the semiconductor optical amplifier is formed in the optical waveguide of the optical circuit element. As a result, an integrated optical element having an external resonator-type light source having favorable characteristics such as polarization dependence is obtained.  
       [0015] In addition, in place of the above-mentioned semiconductor optical amplifier, the integrated optical element according to the present invention may include N (where N is an integer of 2 or more) semiconductor optical amplifiers each having the same structure as the semiconductor optical amplifier, and an optical circuit element including N optical waveguides each corresponding to the associated one of the N semiconductor optical amplifiers. In this case, these N optical semiconductor elements and the optical circuit element are mounted on the element mount surface of the silicon bench via a bonding material. Further, each of gratings with mutually different reflection peak wavelengths is respectively formed in the associated one of N optical waveguides in the optical circuit element. By means of this constitution, an integrated optical element comprising a multi-channel light source (constituted by a plurality of external resonator-type light sources of different oscillation wavelengths) is obtained.  
       [0016] In this case, the optical circuit element may include an optical multiplexer for multiplexing the light propagating through the N optical waveguides.  
       [0017] The interval between the end face of the optical semiconductor element facing the optical waveguide and the optical waveguide of the optical circuit element is preferably filled with resin. As a result, light that is reflected back from the end face of the optical circuit element to the optical semiconductor element is effectively reduced. In such a constitution, the encapsulated resin preferably has a refractive index of 1.300 or more but 1.444 or less, whereby the reflected light is adequately diminished.  
       [0018] Further, the end face of the optical circuit element facing the optical semiconductor element is preferably inclined at an angle of 3° or more but 8° or less with respect to a surface that is orthogonal to the optical axis of the light from the optical semiconductor element. As a result, light that is reflected back from the end face of the optical circuit element to the optical semiconductor element is effectively reduced.  
       [0019] The substrate of the optical circuit element is preferably a silica-based substrate. Because an optical waveguide is thus formed on a silica-based substrate, an optical waveguide having favorable characteristics such as polarization dependence is obtained.  
       [0020] Meanwhile, the optical semiconductor element preferably constitutes a spot size conversion structure (SSC structure) whose FFP (the angle spread of the far field pattern) is 15° or less, and the optical circuit element preferably has a relative refractive index difference between the core and the cladding of the optical waveguide is preferably 1.0% or more. It is therefore possible to match the diameter of the light propagating from the optical semiconductor element to the end face of the optical circuit element, and the mode field diameter (MFD) of the optical waveguide, and, consequently, the efficiency of the coupling between the optical semiconductor element and the optical waveguide can be raised.  
       [0021] Further, according to the fabrication method of the integrated optical element according to the present invention, the glass film of the core and the cladding that constitute the optical waveguide of the optical circuit element are preferably formed by CVD. Because the optical waveguide is formed by CVD with good film thickness control, the alignment accuracy of the optical axis of the optical waveguide with respect to the optical axis of the optical semiconductor element can be improved.  
       [0022] Further, V grooves for mounting the optical fibers to which the light from the optical waveguides in the optical circuit element is input, and alignment marks for recognition by a die bonder when the optical semiconductor element and the optical circuit element are mounted, are preferably formed batchwise on the element mount surface of the silicon bench by means of a KOH etching process. As a result, the accuracy of the mutual alignment between the optical semiconductor element, the optical waveguide of the optical circuit element, and the optical fibers can be raised.  
       [0023] After mounting the optical semiconductor element in a predetermined region on the element mount surface of the silicon bench, the optical circuit element is preferably mounted in a different region from this predetermined region on the element mount surface. As a result, a variation in the characteristics caused by heat generated in the optical circuit element during fabrication of the integrated optical element is effectively suppressed.  
       [0024] The light source module according to the present invention further comprises an integrated optical element that has the structure described above, and outputs light from the light source constituted by the optical semiconductor element and the optical circuit element. In this case, an optical transmission light source module whose light source is an integrated optical element having favorable characteristics such as polarization dependence is obtained.  
       [0025] The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.  
       [0026] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0027]FIG. 1 shows a cross-sectional structure (cross-sectional structure parallel to the direction of light propagation) of a first embodiment of the integrated optical element according to the present invention;  
     [0028]FIG. 2 is a top view showing the planar structure of the integrated optical element according to the first embodiment shown in FIG. 1;  
     [0029]FIG. 3 is a top view that shows the planar structure of the silicon bench of the integrated optical element according to the first embodiment shown in FIG. 1;  
     [0030]FIG. 4 shows a cross-sectional structure (cross-sectional structure perpendicular to the direction of light propagation) of the integrated optical element according to the first embodiment (FIG. 1), along the line I-I in FIG. 2;  
     [0031]FIG. 5 shows a cross-sectional structure (cross-sectional structure perpendicular to the direction of light propagation) of the integrated optical element according to the first embodiment (FIG. 1), along the line II-II in FIG. 2);  
     [0032]FIGS. 6A and 6B are a side view and a top view respectively that show the constitution in which the integrated optical element according to the first embodiment shown in FIG. 1 is filled with resin;  
     [0033]FIGS. 7A to  7 D are process diagrams that serve to illustrate the fabrication method of the integrated optical element according to the first embodiment shown in FIG. 1;  
     [0034]FIGS. 8A to  8 C are graphs showing optical characteristics of the integrated optical element according to the first embodiment shown in FIG. 1;  
     [0035]FIG. 9 is a graph showing coupling loss between the SOA and the optical waveguide of the integrated optical element according to the first embodiment shown in FIG. 1;  
     [0036]FIG. 10 is a graph showing coupling loss between the SOA and the optical waveguide of the integrated optical element according to the first embodiment shown in FIG. 1;  
     [0037]FIG. 11 shows the cross-sectional structure (cross-sectional structure parallel to the direction of light propagation) of a second embodiment of the integrated optical element according to the present invention;  
     [0038]FIG. 12 is a top view showing the parallel structure of the integrated optical element according to the second embodiment arbitrarily shown in FIG. 11;  
     [0039]FIG. 13 is a top view showing a planar structure of a silicon bench of the integrated optical element according to the second embodiment shown in FIG. 11;  
     [0040]FIG. 14 is a partially exploded cross-section showing the constitution of the first embodiment of the light source module according to the present invention; and  
     [0041]FIG. 15 is a perspective view showing the constitution of the second embodiment of the light source module according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0042] Embodiments of the integrated optical element and the like according to the present invention will be described in detail hereinbelow by using FIGS.  1  to  5 ,  6 A to  8 C, and  9  to  15 . In the description of the drawings, the same symbols are assigned to the same elements such that repetitive description is avoided. Further, the dimensional scaling of the drawings does not necessarily match that of the description.  
     [0043]FIG. 1 shows the cross-sectional structure of a first embodiment of the integrated optical element according to the present invention. Further, FIG. 2 is a top view showing the planar structure of the integrated optical element according to the first embodiment shown in FIG. 1. FIG. 1 shows a cross-section that contains the optical axis of a semiconductor optical amplifier  20   1 , an optical waveguide  31   1 , and an optical fiber  40   1  (that will be described later) that is parallel to the direction of light propagation of the integrated optical element. Further, FIG. 3 is a top view that shows the planar structure of the silicon bench of the integrated optical element according to the first embodiment shown in FIG. 1 in a state where the constituent elements of the integrated optical element mounted on the silicon bench are excluded.  
     [0044] An integrated optical element  1 A according to the first embodiment comprises a silicon bench  10  consisting of a silicon (Si) substrate; a semiconductor optical amplifier (SOA)  20 ; an optical circuit element  30 ; and an optical fiber  40 .  
     [0045] The silicon bench  10  comprises an element mount surface for mounting the element chips of the SOA  20  and optical circuit element  30 . The element mount surface of the silicon bench  10  is constituted by a first mount surface  10   a  for mounting the SOA  20 , a second mount surface  10   b  for mounting the optical circuit element  30 , and a third mount surface  10   c  for mounting the optical fiber  40 , moving in a direction from the upstream side to the downstream side in the direction of light propagation. An insulation film is also formed on the element mount surface of the silicon bench  10 .  
     [0046] The SOA  20  is an optical semiconductor element for amplifying light. The integrated optical element  1 A shown in FIGS. 1 and 2 comprises four of the SOA  20 , namely SOA  20   1  to  20   4 . Each of these SOA  20   i  (i=1 to 4) is constituted such that the end face  21  on the upstream side with respect to the direction of light propagation is HR (High-Reflection) coated, and the end face  22  on the downstream side facing the optical circuit element  30  is AR (Anti-Reflection) coated. As a result of this structure, the SOA  20   i  function as optical amplifiers.  
     [0047] These SOA  20   1  to  20   4  are preferably mounted (see FIG. 3) on the first mount surface  10   a  of the silicon bench  10  via bonding pads  51  consisting of AuSn, in a parallel arrangement on the first mount surface  10   a . Further, as shown in FIG. 1, the SOA  20   i  are mounted on the silicon bench  10  such that the light emission layer  26  of the SOA  20   i  is close to the first mount surface  10   a  (such that the stacked film face lying opposite the substrate with the light emission layer  26  interposed therebetween face toward the silicon bench  10 ). Further, alignment marks formed from an electrode material are formed on the stacked film face of the SOA  20   i . Furthermore, an electrode  50  consisting of TiPtAu is preferably provided on the first mount surface  10   a  of the silicon bench  10  whereon the SOA  20   1  to  20   4  are mounted.  
     [0048] The optical circuit element  30  is a planar waveguide-type optical circuit element that comprises a substrate, and an optical waveguide that is provided on the substrate. The optical circuit element  30  comprises a silica-based substrate  35 ; an optical waveguide layer having a predetermined waveguide pattern which is formed on the stacked film face of the silica-based substrate  35 ; and over-cladding  37  that is formed so as to cover the silica-based substrate  35  and optical waveguide layer.  
     [0049] In this first embodiment, the optical waveguide layer on the silica-based substrate  35  is a waveguide pattern that comprises four cores  36  in a mutually parallel arrangement, the direction of light propagation being the longitudinal direction. Accordingly, the optical circuit element  30  comprises four optical waveguides  31   1  to  31   4 . Further, each of these optical waveguides  31   i  (i=1 to 4) is constituted such that the optical axis thereof is provided in a position matching the optical axis of the corresponding SOA  20   i , such that the light from the SOA  20   i  propagates through the optical waveguides  31   i .  
     [0050] Furthermore, optically induced Bragg gratings  32  having a predetermined reflection peak wavelength are formed in the optical waveguides  31   1  to  31   4 . Further, an external resonator-type light source for generating light of a predetermined wavelength is constituted by the SOA  20   i  for amplifying light, and the gratings  32  provided in the associated optical waveguides  31   i . In addition, the gratings  32  provided in the optical waveguides  31   1  to  31   4  have mutually different reflection peak wavelengths. As a result, the integrated optical element  1 A is a four-channel light source that is constituted by four external resonator-type light sources having different oscillation wavelengths.  
     [0051] The optical circuit element  30  comprising these optical waveguides  31   1  to  31   4  is preferably mounted on a second mount surface  10   b  of the silicon bench  10  via a bonding pad  52  consisting of AuSn (see FIG. 3). Further, as shown in FIG. 1, the optical circuit element  30  is mounted on the second mount surface  10   b  such that the optical waveguide layer comprising the cores  36  is located next to the second mount surface  10   b  (such that the stacked film face lying opposite the substrate  35  with respect to the optical waveguide layer are next to the silicon bench  10 ).  
     [0052]FIG. 4 shows the cross-sectional structure (cross-sectional structure perpendicular to the direction of light propagation) of the integrated optical element according to the first embodiment (FIG. 1), along the line I-I in FIG. 2. In this FIG. 4, the cross-sectional structure perpendicular to the direction of light propagation of the integrated optical element  1 A is shown in a position comprising the optical circuit element  30  comprising the optical waveguides  31   1  to  31   4 . As shown in FIGS. 3 and 4, four V grooves  13  are formed in the second mount surface  10   b  of the silicon bench  10 , so as to follow the optical waveguides  31   1  to  31   4 . In addition, a dicing groove  11  is provided in the silicon bench  10 , between the first mount surface  10   a  for mounting the SOA  20   1  to  20   4 , and the second mount surface  10   b  for mounting the optical circuit element  30 .  
     [0053] The optical fiber  40  is an optical waveguide for transmitting light outputted from the SOA  20  and propagates through the optical waveguide  31 . In the first embodiment, four of the optical fiber  40  are arranged, namely optical fibers  40   1  to  40   4 . Each of the optical fibers  40   i  (i=1 to 4) is arranged such that the optical axis of the core  41  thereof is disposed in a position matching the optical axis of the associated optical waveguide  30   i , and the light from the optical waveguides  31   i  is thus input to the optical fibers  40   i .  
     [0054] These optical fibers  40   1  to  40   4  are mounted on the third mount surface  10   c  of the silicon bench  10  in a mutually parallel arrangement.  
     [0055]FIG. 5 shows the cross-sectional structure (cross-sectional structure perpendicular to the direction of light propagation) of the integrated optical element according to the first embodiment (FIG. 1), along the line II-II in FIG. 2). FIG. 5 also shows the optical fibers  40   1  to  40   4 . As shown in FIGS.  3  to  5 , four V grooves  14  are formed in the third mount surface  10   c  of the silicon bench  10 . The optical fibers  40   1  to  40   4  are fixed to the top of the third mount surface  10   c  such that each is aligned by the associated V groove  14 . In addition, a dicing groove  12  is provided in the silicon bench  10 , between the second mount surface  10   b  for mounting the optical circuit element  30 , and the third mount surface  10   c  for mounting the optical fibers  40   1  to  40   4 .  
     [0056] As shown by the solid lines in FIG. 3, the bonding pads  51  for mounting the SOA  20   1  to  20   4  on the silicon bench  10  are provided on the first mount surface  10   a  of the silicon bench  10 . Further, as indicated by the broken lines in FIG. 3, bonding pads  52  for mounting the optical circuit element  30  comprising the optical waveguides  31   1  to  31   4  on the silicon bench  10  are preferably provided via a metal layer consisting of TiPtAu on the surface of the cladding  37  on the optical circuit element  30  side opposite the second mount surface  10   b  of the silicon bench  10 .  
     [0057] In addition, alignment marks  53  for recognition by the die bonder when the SOA  20   1  to  20   4  and optical circuit element  30  are mounted on the element mount surface, are formed on the second mount surface  10   b  of the silicon bench  10 . Likewise, alignment marks  54  are formed on the surface of the cladding  37  of the optical circuit element  30 .  
     [0058] Next, a description will be provided specifically with regard to the effects of the integrated optical element according to the first embodiment.  
     [0059] In order to fabricate the integrated optical element  1 A according to the first embodiment shown in FIGS.  1  to  5 , two types of optical devices, namely the SOA  20   1  to  20   4 , which are optical semiconductor elements, and the optical circuit element  30  comprising the optical waveguides  31   1  to  31   4 , are prepared separately. Further, the integrated optical element  1 A is constituted by mounting the SOA  20   1  to  20   4  and the optical circuit element  30  on the first and second mount surfaces  10   a  and  10   b  respectively of the silicon bench  10 , which are substrates that are provided separately from the substrate  35  of the optical circuit element  30 .  
     [0060] As described above, in this first embodiment, substrates of a suitable material can be applied as the substrate on which the SOA  20   1  to  20   4  are mounted and the substrate whereon the cores  36  and cladding  37  of the optical waveguides  31   1  to  31   4  are formed. Therefore, the integrated optical element  1 A, in which the optical waveguides  31   1  to  31   4  having favorable characteristics such as polarization dependence are integrated with the SOA  20   1  to  20   4 , is obtained. Further, by fabricating the two types of optical devices separately, the fabrication yield of the integrated optical element  1 A can be improved.  
     [0061] As detailed above, a silica-based substrate is preferable for the substrate  35  whereon the cores  36  and cladding  37  of the optical waveguides  31   1  to  31   4  of the optical circuit element  30  are formed. Therefore, because an optical waveguide constituted by a core and cladding is formed on a silica-based substrate, an optical waveguide having favorable characteristics such as polarization dependence is obtained.  
     [0062] When a silica-based substrate is applied as the substrate  35  as described above, the bonding pads  52  for mounting the optical circuit element  30  on the silicon bench  10  are preferably arranged at the four corners of the optical circuit element  30 , as shown in FIG. 3. As a result, contact between the optical circuit element  30  and the silicon bench  10  caused by warping of the substrate  35  is effectively suppressed.  
     [0063] As shown in FIGS. 3 and 4, V grooves  13  are preferably formed in positions corresponding with the second mount surface  10   b  of the silicon bench  10 , with respect to the optical waveguides  31 , to  314  of the optical circuit element  30 . For example, when the cladding  37  is formed by CVD, the surface above the cores  36  of the optical circuit element  30  is sometimes convex. Accordingly, because the V grooves  13  are provided, contact between the convex portion of the optical circuit element  30  and the silicon bench  10  is suppressed, and a contact-induced increase in guided wave loss, and an optical axis displacement, and so forth, are effectively prevented.  
     [0064] Further, in the first embodiment, the SOA  20   1  to  20   4  and the optical circuit element  30 , which are optical semiconductor elements, are arranged such that the light emission layer and optical waveguide layer, respectively, are located next to the element mount surface of the silicon bench  10 . As a result, even in the case of non-alignment, for example, the accuracy of the alignment between the optical axis of the SOA  20   1  to  20   4  and the optical axis of the optical waveguides  31   1  to  31   4  of the optical circuit element  30  improves.  
     [0065] The position of the optical axis in a perpendicular direction can be aligned in accordance with the film stacking accuracy and RIE accuracy, and so forth, when each element is fabricated, and in accordance with the conditions with which the AuSn is heated when each element is mounted via the bonding pads  51  and  52  on the silicon bench  10 .  
     [0066] In this case, the glass films of the cores  36  and cladding  37 , which form the optical waveguides  31   1  to  31   4 , are preferably formed by CVD. In addition, the films for the electrodes consisting of TiPtAu, and so forth, or for the bonding pads consisting of AuSn are preferably formed by vapor deposition. Thus, because films are stacked by using a method with good film thickness control, favorable optical axis alignment accuracy is obtained.  
     [0067] Meanwhile, the position of the optical axis in a horizontal direction can be aligned by allowing the alignment marks  53  and  54  to be distinguished in high accuracy dicing when the SOA  20   1  to  20   4  and the optical circuit element  30  are mounted.  
     [0068] In this case, when the silicon bench  10  is fabricated, the alignment marks  53 , and the V grooves  14  for mounting the optical fibers  40   1  to  40   4  are preferably formed batchwise on the element mount surface using the same photomask by means of a KOH etching process. As a result, displacement between the alignment marks and V grooves is suppressed, and the mutual alignment accuracy of the SOA  20   1  to  20   4 , the optical waveguides  31   1  to  31   4  of the optical circuit element  30   1  and optical fibers  40   1  to  40   4  is raised.  
     [0069] Similarly, also when the optical circuit element  30  is fabricated, the alignment marks  54  are preferably formed by using the same photomask as that for the metal layer. As a result, displacement between the alignment marks and the cores of the optical waveguides is also suppressed.  
     [0070] Further, the shape of each part of the integrated optical element  1 A, and the V groove width, insulation film thickness, electrode thickness, bonding pad thickness, cladding thickness, metal layer thickness, and so forth, for example, are suitably designed for a match between the optical axis of the SOA  20   1  to  20   4 , the optical axis of the optical waveguides  31 , to  314  of the optical circuit element  30 , and the optical axis of the optical fibers  40   1  to  40   4 .  
     [0071] Furthermore, in the first embodiment, the dicing grooves  11  and  12  are provided in the element mount surface of the silicon bench  10 , between the first mount surface  10   a  and second mount surface  10   b , and the second mount surface  10   b  and third mount surface  10   c  respectively. As a result, the introduction of foreign matter between the SOA  20   1  to  20   4  and optical circuit element  30 , and between the optical circuit element  30  and optical fibers  40   1  to  40   4  is prevented.  
     [0072] The film thickness of the electrode  50  formed by TiPtAu or similar on the element mount surface of the silicon bench  10  maybe on the order of 0.56 μm, for example. Further, the film thickness of the bonding pads  51  formed by AuSn or similar maybe on the order of 1.5 μm, for example. When these film thicknesses are too thin, bond strength is not obtained, whereas excessive thickness results in a large optical axis displacement.  
     [0073] The end face of the optical circuit element  30  which faces the SOA  20   i  is preferably inclined at a predetermined angle of 3° or more but 8° or less, for example at an angle of 4.5°, with respect to a surface that is orthogonal to the optical axis of the light from the SOA  20   i  (the surface that is orthogonal to the element mount surface of the silicon bench  10 ) (See the cross-sectional view of FIG. 1). The light reflected back from the end face of the optical circuit element  30  to the SOA  20   i  is thus diminished.  
     [0074] When the inclination angle of the end face of the optical circuit element  30  is greater than 8°, the interval between the SOA  20   i  and the optical circuit element  30  must be widened so that the optical circuit element  30  does not make contact with the SOA  20   i , and hence coupling loss between the optical circuit element  30  and the SOA  20   i  is then large. In addition, when the inclination angle is less than 3°, an adequate reflected light reduction effect is not obtained. Further, in the constitution shown in FIG. 1, the end face of the optical circuit element  30  which faces the optical fiber  40   i  is also formed inclined in the same manner. Further, the distance between the end face of the SOA  20   i  and the end face of the optical circuit element  30  is on the order of 20 μm, for example.  
     [0075] In addition, the SOA  20   i  preferably has a spot size conversion structure (SSC structure) of which the FFP (the angle spread of the far field pattern) is 15° or less, for example 12°. Further, the optical circuit element  30  is preferably constituted such that the relative refractive index difference Δn between the cores  36  and the cladding  37  of the optical waveguides  31   i  is preferably 1.0% or more, for example Δn=1.5%. It is therefore possible to match the diameter of the light propagating from the SOA  20   i  to the end faces of the optical waveguides  31   i  of the optical circuit element  30 , and the mode field diameter (MFD) of the optical waveguides  31   i , and, consequently, a low threshold-value, high-output integrated optical element  1 A in which the efficiency of the coupling between the SOA  20   i  and the optical waveguides  31   i  is high is obtained.  
     [0076] Furthermore, the interval between the end face of the SOA  20   1  to  20   4  next to the optical waveguides  31   1  to  31   4 , and the end face of the optical waveguides  31   1  to  31   4  of the optical circuit element  30  is preferably filled with resin. As a result, the light reflected back from the end face of the optical circuit element  30  to the SOA  20   1  to  20   4  is effectively diminished. Further, because the refractive index of the encapsulated resin is between 1.3 and 1.444, the reflected light is adequately diminished.  
     [0077] As a specific example of the above-described constitution in which resin is encapsulated, the whole of the silicon bench  10 , the SOA  20   1  to  20   4 , and the optical circuit element  30  that constitute the integrated optical element  1 A may be covered by resin  18 , as indicated by the broken lines in the side view of FIG. 6A and the top view of FIG. 6B, for example. Further, other constitutions are also possible. Further, in these constitutions, the AR coat of the downstream side end face  22  of the SOA  20   1  to  20   4  is designed on the basis of the refractive index of the resin  18 .  
     [0078] When light of a 1.55 μm wavelength band is assumed, resin of a refractive index of 1.4 can be employed, for example. When the refractive index of the resin is less than 1.3, the coupling loss at each join end face is then large. Further, when the refractive index is larger than 1.444, in cases where the thickness of the cladding of the optical circuit element is thin, leakage of light occurs next to the resin, and hence the guided wave loss of the optical waveguide increases.  
     [0079] Next, the fabrication method for the integrated optical element  1 A shown in FIGS.  1  to  5  will be described together with a specific constitutional example of the integrated optical element  1 A. FIGS. 7A to  7 D are process diagrams that serve to illustrate the fabrication method of the integrated optical element  1 A according to the first embodiment shown in FIG. 1. Further, each process shown in FIGS. 7A to  7 D is shown by means of the same cross-sectional view as FIG. 1.  
     [0080] First, the silicon bench  10 , which is a substrate for mounting the SOA  20   1  to  20   4 , which are optical semiconductor elements, and the optical circuit element  30 , is fabricated (FIG. 7A). One face of the silicon bench  10  constitutes the element mount surface. The dicing grooves  11  and  12 , V grooves  13  and  14 , an insulation film, the electrode  50  consisting of TiPtAu, and the alignment marks  53  are formed in this element mount surface, and the bonding pads  51  consisting of AuSn, which are for mounting the SOA  20   1  to  20   4 , are formed on the electrode  50 . The thickness of the TiPtAu of the electrode  50  is approximately 0.56 μm, and the thickness of the AuSn of the bonding pads  51  is approximately 1.5 μm. Further, the alignment marks  53  are formed using the same photomask as for the formation of the V grooves  13  and  14  by means of KOH etching.  
     [0081] Next, the four prepared SOA  20   1  to  20   4  are mounted on the first mount surface  10   a  of the element mount surface of the silicon bench  10  (FIG. 7B). When the SOA  20   i  are fabricated, the electrode consisting of Au is formed by means of vapor deposition rather than plating and is formed approximately 1 μm thick. Because the electrode is thus formed by means of vapor deposition, it is possible to reduce a variation in the electrode thickness to about 1 μm±0.1 μm. Further, the SOA  20   i  has an SSC structure, the FFP being 12°.  
     [0082] The SOA  20   1  to  20   4  thus fabricated are loaded onto the first mount surface  10   a  via the bonding pads  51  formed on the silicon bench  10  by using high precision die bonder in a flip chip state where the stacked film layer, whereon the light emission layer  26  and the electrode and so forth are formed, is next to the silicon bench  10 . Here, the SOA  20   1  to  20   4  are secured by fusing together the AuSn of the bonding pads  51  next to the silicon bench  10 , and the Au of the electrode face next to the SOA  20 , to  204 , through the application of heat.  
     [0083] Thereafter, the optical circuit element  30 , which comprises the optical waveguides  31   1  to  31   4 , is mounted on the second mount surface  10   b  of the element mount surface of the silicon bench  10  (FIG. 7C). When the optical circuit element  30  is fabricated, an optical waveguide layer that is 4.5 μm thick is deposited by plasma CVD on a silica-based wafer which is the silica-based substrate  35 . By processing this deposition layer to produce the optical waveguide layer by means of photolithography and RIE to a depth of 4.6 μm, pattern cores  36  that are associated with the four optical waveguides  31   1  to  31   4  are formed. Then, over-cladding  37  that is 12.6 μm thick is deposited by plasma CVD so as to cover the silica-based substrate  35  and the cores  36 .  
     [0084] Here, on account of the optical axis alignment between the SOA  20   i  and the optical waveguides  31   i  of the optical circuit element  30 , the thickness of the over-cladding  37  is about half the thickness (approximately 20 μm) of an ordinary planar optical waveguide. In addition, in order to raise the efficiency of the coupling by matching the SOA  20   i  and the mode field diameter (MFD), the relative refractive index difference between the cores  36  and the cladding  37  of the optical waveguides  31   i  is set such that Δn=1.5%. The interval between adjacent cores  36  is on the order of 500 μm.  
     [0085] A metal layer consisting of TiPtAu that is 0.56 μm thick is formed on the surface of the cladding  37  by means of vapor deposition and lift-off (or vapor deposition, photolithography, and RIE), and bonding pads  52  consisting of AuSn that are 1.5 μm thick are likewise formed on this metal layer by means of vapor deposition and lift-off. By means of this process and by setting the thickness of each layer, it is possible to keep the variation at or less than ±1 μm overall by matching the stacked film thickness and etching depth and so forth of each step. As a result, even when the optical circuit element  30  is mounted on the silicon bench  10  with non-alignment, the efficiency of the coupling between the SOA  20   i  and the optical waveguides  31   i  improves.  
     [0086] This planar waveguide-type optical circuit element  30  is placed on the wafer as is or cut to the appropriate size, and optically induced gratings  32  that have a predetermined reflectance, reflection wavelength bandwidth, and reflection peak wavelength are formed. In addition, in hydrogen processing and annealing, processing is carried out under normal conditions.  
     [0087] Here, the gratings  32  are preferably formed by estimating the amount of the change in the characteristics of the gratings  32  that is caused by the heat, stress and so forth involved in the subsequent bonding and packaging processes. Moreover, the gratings  32  of each of the optical waveguides  31   1  to  31   4  are formed so as to have mutually different reflection wavelength bandwidths and reflection peak wavelengths. After the gratings  32  have been formed, dicing is performed so that the inclination angle of the end face is 4.5°, and the wafer is divided into 2.5 mm×2.5 mm optical circuit element 30 chips.  
     [0088] The optical circuit element  30  that is fabricated as described above is mounted on the second mount surface  10   b  via the bonding pads  52  formed on the optical circuit element  30  by using high precision die bonder in a flip chip state where the stacked film layer, whereon the optical waveguides  31 , to  314 , the bonding pads  52 , and so forth are formed, is next to the silicon bench  10 . Here, the optical circuit element  30  is secured by fusing together the metal layer next to the silicon bench  10 , and the AuSn of the bonding pads  52  next to the optical circuit element  30 , through the application of heat.  
     [0089] In addition, the optical fibers  40   1  to  40   4  are mounted on the third mount surface  10   c  with respect to the silicon bench  10  whereon the SOA  20   1  to  20   4  and the optical circuit element  30  are mounted, whereby the integrated optical element  1 A is obtained (FIG. 7D). Further, where required, a predetermined area that includes the silicon bench  10 , the SOA  20   1  to  20   4 , and the optical circuit element  30  is sealed by means of the resin  18  (See FIGS. 6A and 6B). For example, the integrated optical element  1 A is mounted in a predetermined package, and, after wire bonding and fiber setting, the whole of the integrated optical element  1 A is covered by resin that protects the SOA  20   1  to  20   4  from moisture and so forth. Here, resin is also made to fill the respective intervals between the SOA  20   1  to  20   4  and the optical circuit element  30 , the optical circuit element  30  and the optical fibers  40   1  to  40   4 , and the optical circuit element  30  and the silicon bench  10 , and so forth.  
     [0090] The integrated optical element  1 A that is thus fabricated is constituted as a four-channel light source that comprises a first external resonator-type light source, which comprises the SOA  20   1  and the optical waveguide  31   1  and outputs light of oscillation wavelength λ 1 ; a second external resonator-type light source, which comprises the SOA  20   2  and the optical waveguide  31   2  and outputs light of oscillation wavelength λ 2 ; a third external resonator-type light source, which comprises the SOA  20   3  and the optical waveguide  31   3  and outputs light of oscillation wavelength λ 3 ; and a fourth external resonator-type light source, which comprises the SOA  20   4  and the optical waveguide  31   4  and outputs light of oscillation wavelength λ 4 .  
     [0091] Further, the oscillation wavelengths λ 1  to λ 4  of the integrated optical element  1 A are set by the constitution of the SOA  20   1  to  20   4  and by the constitution of the gratings  32  of the optical waveguides  31   1  to  31   4 , and so forth. In the case of a light source used in the 1.55 μm wavelength band, these oscillation wavelengths are set as λ 1 =1537.2 nm, λ 2 =1543.4 nm, λ 3 =1550.0 nm, and λ 4 =1556.4 nm, for example.  
     [0092]FIGS. 8A to  8 C are graphs showing optical characteristics of the integrated optical element  1 A according to the first embodiment shown in FIG. 1. In particular, FIG. 8A shows light emission spectra for the integrated optical element  1 A, where graphs A 1  to A 4  correspond to the light emission spectra of the above-mentioned first to fourth light sources. Further, FIG. 8B is a graph showing the current-light output characteristic, where graphs B 1  to B 4  correspond to the characteristic of the first to fourth light sources. FIG. 8C is a graph showing the current-oscillation wavelength characteristic, where graphs C 1  to C 4  correspond to the characteristic of the first to fourth light sources.  
     [0093] Here, in case of the fabrication method for the integrated optical element shown in FIGS. 7A to  7 D, the optical circuit element  30  is mounted after the SOA  20   1  to  20   4  have been mounted on the element mount surface of the silicon bench  10 . Because the integrated optical element  1 A is fabricated in this order, degradation of the gratings  32  formed in the optical waveguides  31   1  to  31   4  of the optical circuit element  30  that is caused by the heat involved in mounting is kept to a minimum.  
     [0094] Further, in accordance with this fabrication process, in the first embodiment, the bonding pads  52  for mounting the optical circuit element  30  on the silicon bench  10  are provided next to the optical circuit element  30  rather than next to the silicon bench  10 .  
     [0095] When the bonding pads  52  used to mount the optical circuit element  30  are provided on the element mount surface of the silicon bench  10 , melting takes place as far as the AuSn of the bonding pads  52  due to the heat involved in the mounting of the SOA  20   1  to  20   4 , or deterioration sometimes occurs due to oxidation of the AuSn of the bonding pads  52 . Accordingly, because the bonding pads  52  are provided next to the optical circuit element  30 , the above-described process of sequentially mounting the SOA  20   1  to  20   4  and then the optical circuit element  30  on the silicon bench  10  can be suitably performed.  
     [0096] In addition, the constitution and characteristics of the integrated optical element  1 A according to the first embodiment will now be studied.  
     [0097]FIG. 9 is a graph showing the coupling loss between the SOA  20   i  and optical waveguides  31   i  of the integrated optical element  1 A according to the first embodiment shown in FIG. 1. In this graph, the horizontal axis represents the displacement of axis (μm) of the optical axes of the SOA  20   i  and the optical waveguides  31   i . Further, the vertical axis represents the coupling loss (dB) between the SOA  20   i  and the optical waveguides  31   i .  
     [0098] Further, the distance between the downstream side end face of the SOA  20   i  and the upstream side end face of the optical waveguides  31   i  is 20 μm. Further, for the SOA  20   i , an SOA with an SSC structure of which the FFP is 12° is assumed.  
     [0099] In addition, graphs D 1  to D 4  in FIG. 9 show characteristics of the coupling between the SOA  20   i  and the optical waveguides  31   i  when the relative refractive index difference Δn between the cores  36  and the cladding  37  is changed, in a condition where the core size of the optical waveguides  31   i  that are coupled to the SOA  20   i  is fixed at 4.5 μm×4.5 μm.  
     [0100] More specifically, graph D 1  shows the characteristics when the difference Δn of the optical waveguides  31   i  is 1.50% and the MFD is 5.6 μm. Further, graph D 2  shows the characteristics when Δn is 0.75% and the MFD is 8 μm. Graph D 3  shows the characteristics when Δn is 0.65% and the MFD is 9 μm. Graph D 4  shows the characteristics when Δn is 0.45% and the MFD is 10 μm. Further, the MFD of the SOA  20   i  is 4.8 μm for any of the graphs D 1  to D 4 .  
     [0101] As can be seen from these graphs D 1  to D 4 , when the displacement of axis of the optical axes of the SOA  20   i  and the optical waveguides  31   i  is in the range ±2 μm or less, the coupling loss is kept small when the relative refractive index difference Δn of the optical waveguides  31   i  is large and the MFD is small. Therefore, in the case of the integrated optical element  1 A that comprises the above-described structure, the relative refractive index difference Δn between the cores  36  and cladding  37  of the optical waveguides  31   i  of the optical circuit element  30  is preferably set at 1.0% or more.  
     [0102]FIG. 10 is a graph showing coupling loss between the SOA  20   i  and the optical waveguides  31   i  of the integrated optical element  1 A according to the first embodiment shown in FIG. 1. In this graph, the horizontal axis represents the displacement of axis (um) of the optical axes of the SOA  20   i  and the optical waveguides  31   i , and the vertical axis represents the coupling loss (dB) between the SOA  20   i  and the optical waveguides  31   i .  
     [0103] Here, the distance between the downstream side end face of the SOA  20   i  and the upstream side end face of the optical waveguides  31   i  is 20 μm. Further, for the optical waveguides  31   i , an optical waveguide for which the relative refractive index difference Δn between the cores  36  and the cladding  37  is 1.5% and the MFD is 5.6 μm is assumed.  
     [0104] Further, graphs E 1  to E 3  show the coupling characteristics when the FFP of the SOA  20   i  with an SSC structure is changed.  
     [0105] In particular, graph E 1  shows the characteristics when the FFP of the SOA  20   i  is 12°. Further, graph E 2  shows the characteristics when the FFP of the SOA  20   i  is 16°. Graph E 3  shows the characteristics when the FFP of the SOA  20   i  is 20°.  
     [0106] As shown in these graphs E 1  to E 3 , the coupling loss is kept small when the FFP of the SOA  20   i  with the SSC structure is small. Therefore, in the case of the integrated optical element  1 A that has the above-described structure, the FFP with this SSC structure is preferably set at 15° or less.  
     [0107]FIG. 11 shows the cross-sectional structure of the second embodiment of the integrated optical element according to the present invention. Further, FIG. 12 is a top view showing the parallel structure of the integrated optical element according to the second embodiment shown in FIG. 11. FIG. 11 shows a cross-section that contains the optical axes of the SOA  20   1  and optical waveguide  31   1  that is parallel to the direction of light propagation of the integrated optical element.  
     [0108] In addition, FIG. 13 is a top view showing a planar structure of a silicon bench of the integrated optical element shown in FIGS. 11 and 12. The constituent elements of the integrated optical element mounted on the silicon bench are excluded from FIG. 13.  
     [0109] An integrated optical element  1 B according to the second embodiment comprises the silicon bench  10 , the SOA  20 , the optical circuit element  30 , and the optical fiber  40 .  
     [0110] The first mount surface  10   a  for mounting the SOA  20 , the second mount surface  10   b  for mounting the optical circuit element  30 , and the third mount surface  10   c  for mounting the optical fiber  40  are provided on the element mount surface of the silicon bench  10 , moving in a direction from the upstream side to the downstream side in the direction of light propagation. An insulation film is also formed on the element mount surface of the silicon bench  10 .  
     [0111] The integrated optical element  1 B shown in FIGS. 11 and 12 is provided with four of the SOA  20 , namely SOA  20   1  to  20   4 . Each of these SOA  20   i  (i=1 to 4) is constituted such that the upstream side end face  21  thereof is HR coated, and the downstream side end face  22  thereof is AR coated. As a result, the SOA  20   i  function as optical amplifiers.  
     [0112] These SOA  20   1  to  20   4  are mounted (see FIG. 13) in a parallel arrangement on the first mount surface  10   a  of the silicon bench  10  via bonding pads  51 . Further, as shown in FIG. 11, the SOA  20   i  are mounted in a flip chip state such that the light emission layer  26  of the SOA  20   i  is located next to the first mount surface  10   a . Further, alignment marks formed from an electrode material are formed on the stacked film face of the SOA  20   i . An electrode  50  is provided on the first mount surface  10   a  of the silicon bench  10  whereon the SOA  20   1  to  20   4  are mounted.  
     [0113] The optical circuit element  30  comprises a silica-based substrate  35 ; an optical waveguide layer formed by a predetermined waveguide pattern on the stacked film face of the silica-based substrate  35 ; and over-cladding  37  that is formed so as to cover the silica-based substrate  35  and optical waveguide layer.  
     [0114] In this second embodiment, the optical waveguide layer on the silica-based substrate  35  in the upstream side part of the optical circuit in the optical circuit element  30  is formed by a waveguide pattern that comprises four cores  36  in a mutually parallel arrangement, the direction of light propagation being the longitudinal direction. Accordingly, upstream side part of the optical circuit element  30  comprises four optical waveguides  31   1  to  31   4 . Further, each of these optical waveguides  31   i  (i=1 to 4) is constituted such that the optical axis thereof is provided in a position matching the optical axis of the corresponding SOA  20   i , such that the light from the SOA  20   i  propagates through the optical waveguides  31   i .  
     [0115] Furthermore, optically induced Bragg gratings  32  having a predetermined reflection peak wavelength are formed in the optical waveguides  31   1  to  31   4 . Further, an external resonator-type light source for generating light of a predetermined wavelength is constituted by the SOA  20   i  for amplifying light, and the gratings  32  provided in the associated optical waveguides  31   i . In addition, the gratings  32  provided in the optical waveguides  31   1  to  31   4  have mutually different reflection peak wavelengths. As a result, the integrated optical element  1 B of the second embodiment is a four-channel light source that is constituted by four external resonator-type light sources having different oscillation wavelengths.  
     [0116] Meanwhile, the optical waveguide layer on the silica-based substrate  35  in the downstream side part of the optical circuit in the optical circuit element  30  is formed by a waveguide pattern that comprises an optical multiplexer  33 , and an output optical waveguide  34 . The optical waveguides  31   1  to  31   4  on the upstream side are each connected to the optical multiplexer  33 . The optical multiplexer  33  multiplexes the four channels that are input via the optical waveguides  31   1  to  31   4  and outputs the multiplexed channels to the optical waveguide  34 .  
     [0117] Further, an AWG (Arrayed Waveguide Grating), an MZI (Mach-Zehnder Interferometer) or an MMI (Multimode Interference) coupler and so forth, for example, can be applied as the optical multiplexer  33  shown in FIG. 12.  
     [0118] The optical circuit element  30 , which comprises the optical waveguides  31   1  to  31   4 , optical multiplexer  33 , and optical waveguide  34 , is mounted on the second mount surface  10   b  of the silicon bench  10  via the bonding pads  52  (See FIG. 13). Also, as shown in FIG. 11, the optical circuit element  30  is mounted in a flip chip state such that the optical waveguide layer comprising the cores  36  is located next to the second mount surface  10   b.    
     [0119] As shown in FIG. 13, four V grooves  13  that follow the optical waveguides  31   1  to  31   4  are formed in the second mount surface  10   b  of the silicon bench  10 , and a V groove  15  is formed so as to follow the output optical waveguide  34 . In addition, a dicing groove  16  is provided in the second mount surface  10   b , in the area including the part facing the optical multiplexer  33 . The dicing groove  11  is provided in the silicon bench  10 , between the first mount surface  10   a  for mounting the SOA  20 , to  204 , and the second mount surface  10   b  for mounting the optical circuit element  30 .  
     [0120] In the second embodiment, one optical fiber  40  is provided. This optical fiber  40  is constituted such that the optical axis of the core  41  is provided in a position matching the optical axis of the associated optical waveguide  34 , such that light from the optical waveguide  34  is input to the optical fiber  40 .  
     [0121] The optical fiber  40  is mounted on the third mount surface  10   c  of the silicon bench  10 .  
     [0122] As shown in FIG. 13, the V groove  14  is formed in the third mount surface  10   c  of the silicon bench  10 . The optical fiber  40  is aligned by the associated V groove  14 . In addition, a the dicing groove  12  is provided in the silicon bench  10 , between the second mount surface  10   b  for mounting the optical circuit element  30  and the third mount surface  10   c  for mounting the optical fiber  40 .  
     [0123] As indicated by the solid lines in FIG. 13, the bonding pads  51  for mounting the SOA  20 , to  204  on the silicon bench  10  are provided on the first mount surface  10   a  next to the silicon bench  10 . Further, the bonding pads  52 , which serve to mount the optical circuit element  30  comprising the optical waveguides  31 , to  314 , optical multiplexer  33 , and optical waveguide  34  on the silicon bench  10 , are provided, via a metal layer, on the surface of the cladding  37  next to the optical circuit element  30  which faces the second mount surface  10   b  of the silicon bench  10 , as indicated by the broken lines in FIG. 13.  
     [0124] The alignment marks  53 , which are recognized by a die bonder when the SOA  20   1  to  20   4  and optical circuit element  30  are mounted on the element mount surface, are formed on the second mount surface  10   b  of the silicon bench  10 . Likewise, alignment marks  54  are formed on the surface of the cladding  37  of the optical circuit element  30 .  
     [0125] Next, the effects of the integrated optical element according to the second embodiment will be described.  
     [0126] Two types of optical devices, namely the SOA  20   1  to  20   4  and the optical circuit element  30 , are used separately in the fabrication of the integrated optical element  1 B according to the second embodiment shown in FIGS.  11  to  13 . Further, the integrated optical element  1 B is obtained by mounting the SOA  20   1  to  20   4  and the optical circuit element  30  on predetermined surfaces of the silicon bench  10  that are provided separately from the substrate  35  for the optical circuit element  30 . As a result, the integrated optical element  1 B, in which optical waveguides  31   1  to  31   4  having favorable characteristics such as polarization dependence are integrated with the SOA  20   1  to  20   4 , is obtained. In addition, because optical devices of two types are fabricated separately, the fabrication yield of the integrated optical element  1 B increases rapidly.  
     [0127] Furthermore, for the constitution of the optical circuit element  30 , in addition to the constitution of the integrated optical element  1 A according to the first embodiment shown in FIG. 1, it is possible to employ an optical circuit element in which an optical waveguide is formed by an optical circuit pattern that comprises the optical multiplexer  33  as per the second embodiment. In this constitution, outputs can be made from a single optical fiber  40  by multiplexing the four channels generated.  
     [0128] Next, a description will be provided for a light source module in which an integrated optical element with the above-described structure (the integrated optical element according to the present invention) is applied.  
     [0129]FIG. 14 is a partially exploded cross-section showing the constitution of the first embodiment of the light source module according to the present invention. The light source module  6  according to the first embodiment is an optical module in which the integrated optical element  1 B shown in FIG. 11 that constitutes a four-channel light source is installed in a substantially cylindrical housing  60 . In the integrated optical element  1 B, the light of four channels that is generated by the SOA  20   1  to  20   4  and the optical waveguides  31   1  to  31   4  is multiplexed by the optical multiplexer  33 , and then output via the optical fiber  40 .  
     [0130] A ferrule  61 , a lens  63 , and the integrated optical element  1 B are installed so as to achieve a match between the optical axes thereof, in the housing  60  of the light source module  6 . The integrated optical element  1 B is installed on the base  65  of the housing  60  such that the SOA  20   1  to  20   4  are located next to the base  65  and the optical fiber  40  is located next to the lens  63 . In addition, pins  66  for supplying the required electrical signals and so forth to the elements of the integrated optical element  1 B are provided in the base  65 .  
     [0131] In the above constitution, light that is output by the integrated optical element  1 B is input to an optical fiber  62  that passes through the ferrule  61 , via a condenser lens  63 , and is output to the outside via this optical fiber  62 .  
     [0132]FIG. 15 is a perspective view showing the constitution of the second embodiment of the light source module according to the present invention. A light source module  7  according to the second embodiment is an optical module in which the integrated optical element  1 B shown in FIG. 11 that constitutes a four channel light source is installed in a substantially square-shaped package  70 .  
     [0133] A ferrule  71  and the integrated optical element  1 B are installed so as to achieve a match between the optical axes thereof, in the package  70  of the light source module  7 . The integrated optical element  1 B is installed such that the SOA  20 , to  204  are located on the opposite side from the ferrule  71  and the optical fiber  40  is located next to the ferrule  71 , on the bottom  75  of the package  70 . Further, the optical fiber  40  is connected to an optical fiber  72  that passes through the ferrule  71 . Pins  76  for supplying the required electrical signals and so forth to the elements of the integrated optical element  1 B are provided in a surface next to the SOA  20 , to  204  of the package  70 .  
     [0134] In the above constitution, the light output from the integrated optical element  1 B is input via the optical fiber  40  to the optical fiber  72  that passes through the ferrule  71  and then output to the outside via the optical fiber  72 .  
     [0135] As in the case of the light source modules  6  and  7  of FIGS. 14 and 15, an optical transmission light source module whose light source is an integrated optical element having favorable characteristics such as polarization dependence is obtained by using the integrated optical element of the above-described constitution to output light from the light source constituted by the optical semiconductor element and the optical circuit element that are mounted on the silicon bench. Further, in the case of this light source module, the integrated optical element  1 A shown in FIG. 1 may be applied.  
     [0136] The integrated optical element, fabrication method for this integrated optical element, and light source module according to the present invention are not limited to or by the above embodiments, a variety of modifications being possible. For example, in the case of the integrated optical elements  1 A and  1 B shown in FIGS. 1 and 11, the semiconductor optical amplifiers  20   1  to  20   4  are used as optical semiconductor elements, and the gratings  32  are formed in the optical waveguides  31   1  to  31   4 , whereby an external resonator-type light source is formed. Accordingly, the constitution may be one in which a semiconductor laser element is used as the optical semiconductor element and gratings are not formed in the optical waveguides.  
     [0137] Moreover, although both the integrated optical elements  1 A and  1 B are constituted as a four channel light source, generally, the integrated optical elements  1 A and  1 B can be a light source with one or more channel (s) constituted by one or more optical semiconductor element(s) and optical waveguide(s). With regard to the mounting of the elements on the silicon bench, a mounting method other than the flip chip mounting method is acceptable depending on the alignment accuracy and so forth required.  
     [0138] According to the invention described hereinabove, optical semiconductor elements for outputting light of a predetermined wavelength, and an optical circuit element in which optical waveguides that propagate the light from the optical semiconductor elements are formed on a substrate, are mounted on a separately prepared silicon bench via a bonding material. For this reason, substrates of suitable materials can be used as the substrate whereon the optical semiconductor elements are mounted and the substrate for the optical circuit element on which the optical waveguides are formed.  
     [0139] Therefore, an integrated optical element, in which an optical waveguide having favorable characteristics such as polarization dependence is integrated with an optical semiconductor element, and a fabrication method for the integrated optical element, are obtained. Furthermore, because optical devices of two types are fabricated separately, the fabrication yield of the integrated optical element can be improved.  
     [0140] Moreover, in the case of the light source module that comprises the above-described integrated optical element and that outputs light from the light source constituted by the optical semiconductor element and the optical circuit element, an optical transmission light source module whose light source is an integrated optical element having favorable characteristics such as polarization dependence is obtained.  
     [0141] From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.