Abstract:
An objective is to provide an integrated optical module which can avoid positional change and separation of a PLC chip when humidity changes. Provided is an integrated optical module characterized in that the integrated optical module includes: a PLC chip; a seat bonded and fixed to part of a lower surface of the PLC chip with an adhesive which is applied to an upper surface of the seat; and a support portion supporting the seat, in which a groove where an adhesive overflowing from the upper surface of the seat is to stay is formed in an upper surface of the support portion at a portion surrounding the seat, the upper surface of the seat serving as an adhesion surface.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an integrated optical module, and relates to an integrated optical module mounted with a planar light circuit which is integrated together with a light emitting element or a light receiving element and forms an optical transmitter or receiver. 
       BACKGROUND ART 
       [0002]    With the development of optical communication technology, development of optical components is becoming more and more important. Above all, an optical transmitter or receiver is increased in its transmission speed and response speed, and has a larger communication capacity. In a configuration of a typical transmitter or receiver, the transmitter or receiver includes a light emitting element or a light receiving element fabricated using optical semiconductor, and an output or input optical fiber, and they are optically coupled to each other via a lens. For example, in a case of an optical receiver, light emitted from the input optical fiber is focused on the light receiving element by the lens, and is directly detected (intensity detection). 
         [0003]    Turning to a modulation and demodulation processing technique for an optical transmission system, signal transmission using a phase modulation method is in wide practical use. A phase shift keying (PSK) method is a method for transmitting a signal through modulation of the phase of light. The PSK method can achieve much larger transmission capacity than before by way of multi symbol modulation or the like. 
         [0004]    In order to receive such a PSK signal, the phase of light needs to be detected. The light receiving element can detect the intensity of signal light, but cannot directly detect the phase of the light. Hence, a means for converting the phase of light into light intensity is needed. For example, there is a method for detecting phase difference by using interference of light. Information on the phase of light can be obtained by causing signal light to interfere with other light (reference light) and detecting the light intensity of interfering light. There are coherent detection and differential detection. In the coherent detection, a light source prepared separately is used as the reference light. In the differential detection, part of signal light is branched off and is used as reference light, and the signal light is caused to interfere with the reference light. As described above, unlike a conventional optical receiver using only an intensity modulation method, a recent optical receiver using the PSK method needs an optical interference circuit which converts phase information into intensity information through interference of light. 
         [0005]    Such an optical interference circuit can be achieved using a planar light circuit (PLC). The PLC delivers superior features in terms of mass productivity, low cost, and high reliability, and can be used as various types of light interference circuit. In fact, as an optical interference circuit used in a PSK optical receiver, an optical delay interference circuit, a 90-degree hybrid circuit, and the like are offered and in practical use. Such a PLC is fabricated by a glass deposition technique such as standard photography, etching, and FHD (Flame Hydrolysis Deposition). 
         [0006]    In a specific manufacturing process, first, an under-cladding layer made mainly of silica glass or the like and a core layer having a higher refractive index than the under-cladding layer are deposited on a substrate made of Si or the like. Thereafter, various patterns of waveguides are formed on the core layer. Lastly, the waveguides are embedded by an over-cladding layer. A PLC chip having a waveguide-type optical functional circuit is fabricated by such a process. Signal light is encapsulated in the waveguides fabricated by the above process and propagated within the PLC chip. 
         [0007]      FIG. 1  shows a conventional method for optically connecting a PLC and an optical receiver. Simple fiber connection as shown in  FIG. 1  is employed as a basic method for connection of a PLC and an optical receiver in a PSK optical receiver. Optical coupling is established by connecting a planar light circuit (PLC)  1 , which is connected at its input end to an optical fiber  3   a,  and an optical receiver  2  to each other with optical fibers  3   b.  The number of optical fibers  3   b  used for the optical coupling is determined by the number of output light beams outputted from the PLC. Multiple optical fibers are used for the optical coupling in some cases. For this reason, such a configuration of an optical receiver using optical fiber connection may have too large a size. To overcome such a problem in the configuration, the optical receiver can be reduced in size by coupling the output of the PLC and the input of the optical receiver with no optical fiber interposed therebetween and by integrating all into one package. Such a form of an optical receiver in which the PLC and the optical receiver are optically coupled together directly is called an integrated optical module. 
         [0008]    To obtain an integrated optical module, how to fix the PLC chip is particularly important. In a case of optically coupling light outputted from the PLC chip and propagated in airspace to a light receiving element by a lens or the like, if the positional relation among the end of light emission from the PLC chip, the lens, and the light receiving element changes, not all of the light can be received by the light receiving element, causing a loss. Such a loss problem is especially noticeable when ambient temperature changes to change the temperature of the package housing the optical receiver, the temperature of each element, and the like, and their positions change due to the influence of thermal expansion. To achieve optical coupling with low loss, it is necessary that the positional relation among the components does not change, at least not relative to each other, even if ambient temperature or the like changes. 
         [0009]    In particular, the PLC chip occupies more area in the optical receiver than the light receiving element by about one to two digits, and is therefore more likely to change in shape due to the thermal expansion. Further, a substrate and a deposited thin-film glass which constitute the planar light circuit are largely different in their coefficients of thermal expansion, and therefore temperature change causes large warpage. For this reason, changes in the position and angle of light emitted from the PLC chip relative to the light receiving element are really problematic. These two changes cause the position and angle of light emitted from the planar light circuit to change, leading to displacement in the optical axis. The displacement in the optical axis deteriorates optical coupling of the PLC chip to the light receiving element, and causes a loss. In order to achieve an integrated optical module, it is important to overcome such displacement in the optical axis or to render the displacement harmless. 
         [0010]      FIG. 2  shows an internal structure of a conventional integrated optical module. There is known a method for securely fixing the almost entire bottom surface of the PLC chip so that the aforementioned optical-axis displacement may not occur when temperature changes. In the integrated optical module shown in  FIG. 2 , a PLC chip  13  in which an optical interference circuit is formed as an optical functional circuit, a lens  14 , and a light receiving element  15  are fixed to a base substrate  11  with fixing mounts  12   a,    12   b,    12   c  as support members, respectively. An optical fiber  16  and the PLC chip  13  are connected to each other via an optical-fiber fixing component  17 . In this integrated optical module, light inputted from the optical fiber  16  interferes in the PLC chip  13 , and is then coupled to the light receiving element  15  by the lens  14 . 
         [0011]    The fixing mount  12   a  and the PLC chip  13  are fixed together by an adhesive  18  or solder. The almost entire bottom surface of the PLC chip  13  is securely fixed to the fixing mount, so that temperature-related expansion or warpage is suppressed. Further, the lens  14  and the light receiving element  15  are also fixed to their fixing mounts, so that the optical axis may not be displaced when temperature changes. 
         [0012]    The configuration shown in  FIG. 2  can suppress or sufficiently reduce the optical-axis displacement caused by temperature change, but makes noticeable the change in the properties of the PLC chip due to temperature change. As described earlier, the planar light circuit  13  includes a Si substrate  13   a  and a silica glass layer  13   b  which are largely different in their coefficients of thermal expansion, and are likely to suffer from great warpage or thermal expansion when temperature changes. In the configuration shown in  FIG. 2 , thermal expansion and warpage are suppressed because the entire bottom surface of the PLC chip  13  is fixed. 
         [0013]    On the other hand, in this case, a large thermal stress is generated between the Si substrate  13   a  and the silica glass layer  13   b.  This stress causes change in the refractive index in the silica glass layer  13   b  through a photo-elastic effect. In the light interference circuit formed in the PLC chip  13 , the lengths of waveguides and the refractive indices are precisely adjusted in order to control interference property. The change in the refractive index caused by the stress brings about a change in an equivalent circuit length to change the properties of an interferometer, and consequently, deteriorates the properties of the optical interference circuit. 
         [0014]    If, in order to suppress the change in optical properties by suppressing the occurrence of thermal stress, an elastic adhesive, a soft adhesive such as paste, or a fixing paste is used as the adhesive  18  (see, for example, PTL 1), the aforementioned influence on the optical-axis displacement becomes noticeable, and this causes loss. 
       CITATION LIST 
     Patent Literature 
       [0015]    PTL 1: Japanese Patent Laid-Open No. 2009-175364 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0016]    To solve the above problems, a configuration shown in  FIG. 3  has been proposed to be employed in an integrated optical module in which optical components such as a PLC chip are integrated. In this configuration, an adhesive  38  is applied to a seat formed by raising part of a fixing mount  32   a,  and a PLC chip  33  is bonded and fixed to the base. Other configurations in  FIG. 3  are similar to those in  FIG. 2 . Specifically, an optical fiber  36  is connected to the PLC chip  33  via an optical-fiber fixing component  37 , and optical components such as the PLC chip  33 , a lens  34 , and a light receiving element  35  are mounted on a base substrate  31  via fixing mounts  32   a,    32   b,    32   c.  By such a configuration, even if deformation or warpage occurs in the PLC chip due to temperature change, the optical functional circuit is affected by the stress only to a minimum degree. Thus, deterioration in the properties of the optical functional circuit can be suppressed. 
         [0017]    However, in this integrated optical module, as shown in  FIG. 4A , if the adhesive  38  for connecting the PLC chip  33  to a seat portion  42  of a mount  40  is applied too much, the adhesive  38  overflows around the seat portion  42  and hardens. The overflowing adhesive  38  is likely to concentrate around the seat by its adhesive property, and does not spread over the mount but stays around the adhesion surface from which the adhesive  38  has overflowed.  FIG. 4B  is a top view of the mount. As shown in  FIG. 4B , the seat  42  of square section is provided on part of the mount. Thus, the overflowing adhesive stays along the four sides of the seat. Since the adhesion surface between the seat  42  and the PLC chip  33  is very small, in order to control the amount of the adhesion precisely, the amount has to be controlled on a μl level. In addition, generally, metals do not change much in volume when humidity changes, but resins change in volume much when humidity changes. When humidity changes, the adhesive  38  made of a resin swells, but the seat  42  of the mount  40  made of a metal does not change in volume. Hence, if humidity changes with the overflowing adhesive staying around the adhesion surface, the overflowing adhesive  38  swells to generate force F pushing up the PLC chip  33 . This serves as a cause for positional change and separation of the PLC chip. Note that the section of the seat referred to herein is a section viewed from the upper surface of a support portion  41  of the mount  40 . 
         [0018]    In view of above, the present invention has an objective of providing an integrated optical module which can avoid positional change and separation of a PLC chip when humidity changes. 
       Solution to Problem 
       [0019]    To solve the above problem, an invention described in one embodiment provides an integrated optical module characterized in that the integrated optical module comprises: a PLC chip; a seat bonded and fixed to part of a lower surface of the PLC chip with an adhesive which is applied to an upper surface of the seat; and a support portion supporting the seat, in which a groove where an adhesive overflowing from the upper surface of the seat is to stay is formed in an upper surface of the support portion at a portion surrounding the seat, the upper surface of the seat serving as an adhesion surface. 
         [0020]    In the above integrated optical module, part of the groove preferably has a penetrating hole penetrating from the upper surface to a lower surface of the support portion. 
         [0021]    In the above integrated optical module, a shape of the groove preferably is symmetric around the seat. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0022]      FIG. 1  is a diagram showing a conventional method of optical connection between a planar light circuit and an optical receiver; 
           [0023]      FIG. 2  is a diagram showing an internal structure of an example of a conventional integrated optical module; 
           [0024]      FIG. 3  is a diagram showing an internal structure of another example of a conventional integrated optical module; 
           [0025]      FIG. 4A  is a diagram illustrating positional change and separation of the PLC chip in the conventional integrated optical module; 
           [0026]      FIG. 4B  is a diagram illustrating positional change and separation of the PLC chip in the conventional integrated optical module; 
           [0027]      FIG. 5A  is aside sectional view showing an internal structure of an example of an integrated optical module of a first embodiment; 
           [0028]      FIG. 5B  is a top view showing the internal structure of the example of the integrated optical module of the first embodiment; 
           [0029]      FIG. 6A  is aside sectional view showing an internal structure of an example of an integrated optical module of a second embodiment; and 
           [0030]      FIG. 6B  is a top view showing the internal structure of the example of the integrated optical module of the second embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0031]    Embodiments of the present invention are described in detail below. 
       First Embodiment 
       [0032]      FIGS. 5A and 5B  are each a diagram showing a main portion of an integrated optical module of a first embodiment.  FIG. 5A  is a side sectional view showing a schematic configuration of the main portion of the integrated optical module of the first embodiment, and  FIG. 5B  is a top view of a mount used in the integrated optical module of the first embodiment. The integrated optical module is configured such that optical components, such as a PLC chip, a lens, and a light receiving element or a light emitting element, are mounted on a base substrate via mounts, and these components are sealed by a package. As shown in  FIG. 5A , a PLC chip  33  on which an optical interference circuit is formed is connected to an optical fiber  36  via a fiber-fixing component  37 , and is bonded and fixed to a mount  40  with an adhesive  38   a.    
         [0033]    The PLC chip  33  is formed such that a silica glass layer  33   b  is stacked on a Si substrate  33   a.  The silica glass layer  33   b  has formed thereon a waveguide-type optical functional circuit formed by a core and claddings. The adhesive  38  may be, for example, any of an epoxy adhesive that hardens with heat, an adhesive that hardens with moisture, and an adhesive that hardens with oxygen. 
         [0034]    The mount  40  can be formed from a metal such as Kovar. The mount  40  includes a plate-shaped support portion  41  to be mounted on the base substrate, a seat  42  formed by raising part of an upper surface of the plate-shaped support portion  41 , and a groove portion  43  provided in the support portion  41  at a portion surrounding the seat  42 . The adhesive  38   a  is applied to an adhesion surface which is an upper surface of the seat  42  of the mount  40 , and the PLC chip  33  is bonded and fixed at part of its lower surface. In the integrated optical module of the present invention, the groove portion  43  is formed around the seat  42  which is formed as part of the mount  40 , so as to accommodate an adhesive  38   b  overflowing from the adhesion surface between the PLC chip  33  and the mount  40 . While the mount  40  is made of a metal, the adhesive is made of a resin. Thus, only the adhesive swells when the humidity increases. Since the groove portion  43  is formed around the seat  42 , the adhesive  38   b  overflowing from the adhesion surface is accommodated in the groove portion  43 . Thus, the adhesive  38   b  does not exert a pressure pushing up the PLC chip  33  when swelling, and therefore, positional change and separation of the PLC chip  33  do not occur. 
         [0035]    A volume V 2  of the groove portion  43  is determined based on the allowable amount of adhesive. For example, by setting the volume V 2  to a value larger than an amount V 1  of the adhesive  38   a  needed by the adhesion surface between the PLC chip  33  and the seat  40 , even if the adhesive is applied twice or more than twice the necessary amount V 1  of the adhesive is applied, the overflowing adhesive  38   b  does not exert a pressure pushing up the PLC chip  33 . 
         [0036]    The groove portion  43  can be formed by use of a cutting drill. The groove portion  43  can be formed to have a width of, for example, 1 mm. Although the width of the groove portion  43  does not have to be constant, it is preferable that the groove portion  43  be formed such that four sides around the seat  42  are symmetric. The width of the groove portion  43  is preferably small because the adhesive is then permitted to enter the groove portion  43  due to capillary action. However, the groove portion  43  needs to have a certain width in order to accommodate a certain amount of overflowing adhesive. On the other hand, if the groove portion  43  is too wide and if the surface of the support portion  41  of the mount  40  has poor wettability, the adhesive may not enter the groove portion  43 . For this reason, the width of the groove portion  43  is determined according to a relation between the wettability of the surface of the mount  40  and the surface tension of the adhesive. The groove portion  43  does not need to be provided along the entire periphery of the seat  42 , and may be provided in only part of each of the surrounding four sides. In this case, it is preferable that each portion of the groove portion  43  is provided such that the groove portion  43  is symmetric in shape with the four sides. 
         [0037]    The sectional shape of the seat  42  is not limited to a square as shown in  FIG. 5B , but may be any shape such as a circle. However, if the section is square as shown in  FIG. 5B , a large adhesive area can be obtained to achieve stable adhesion between the seat  42  and the PLC chip  33 . 
         [0038]    According to the embodiment described above, the groove portion  43  is formed around the seat  42  which is formed as part of the mount  40  so as to be able to accommodate the adhesive  38   b  overflowing from the adhesion surface between the PLC chip  33  and the mount  40 . Thus, the overflowing adhesive  38   b  does not exert a pressure pushing up the PLC chip  33  when swelling, and therefore, positional change and separation of the PLC chip  33  do not occur. 
       Second Embodiment 
       [0039]      FIGS. 6A and 6B  are diagrams each showing a main portion of an integrated optical module of a second embodiment.  FIG. 6A  is a side sectional view showing a schematic configuration of the main portion of the integrated optical module of the second embodiment, and  FIG. 6  is a top view of a mount used in the integrated optical module of the second embodiment. The integrated optical module of this embodiment has the same configuration as the integrated optical module of the first embodiment, except that part of the groove portion  43  is formed as a penetrating hole  44  penetrating the support portion  41 . 
         [0040]    In the integrated optical module of this embodiment, part of the groove portion  43  is formed as the penetrating hole  44 . The penetrating hole  44  enables observation of how far the adhesive flows and what kind of adhesion state is caused by how much adhesive. In a conventional module, the state of the adhesion needs to be observed by checking the seat portion laterally through a gap between the PLC chip  33  and the support portion  41  of the mount  40  of the module. This gap portion is about several hundred μm and very small, making the observation really difficult. 
         [0041]    The penetrating hole  44  can be formed using a cutting drill, like the groove  43 . The penetrating hole  44  may be formed in part of each side of the groove formed along the four sides. Preferably, the shape of the penetrating hole  44  in each side is the same. This is because deformation can be prevented by the symmetry. 
         [0042]    By the provision of the penetrating hole  44 , even if the overflowing adhesive does not enter the groove portion  43  due to the groove portion  43  being too wide or the mount  40  having poor wettability, excessive adhesive can be removed from the outside by vacuum suction through the penetrating hole  44  from a surface of the mount  40  which is opposite from the adhesion surface. 
         [0043]    As described, according to the configuration of this embodiment, the penetrating hole is formed in part of the groove portion  43  formed around the seat  42  which is formed as part of the mount  40  so as to be able to accommodate the adhesive  38   b  overflowing from the adhesion surface between the PLC chip  33  and the mount  40 . Thus, the overflowing adhesive  38  does not exert a pressure pushing up the PLC chip  33  when swelling, and therefore, positional change and separation of the PLC chip  33  do not occur. 
         [0044]    Although the support portion and the seat which constitute the mount are integrally formed in the above embodiments as an example, the seat may be bonded to the upper surface of the support portion with an adhesive. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           33  PLC chip 
           33   a  Si substrate 
           33   b  silica glass layer 
           36  optical fiber 
           37  fiber-fixing component 
           38   a,    38   b  adhesive 
           40  mount 
           41  support portion 
           42  seat 
           43  groove portion 
           44  penetrating hole