Abstract:
An optical waveguide and a first lens are formed on an underlying surface. The optical waveguide guides light along a first direction. The first lens is continuous with one end of the waveguide and converges light radiated from the end plane of the optical waveguide and diverging along directions parallel to the underlying surface. A second lens converges light transmitted through the first lens and diverging along directions perpendicular to the underlying surface. A support member supports the first and second lenses. It is possible to prevent a shift of positions of the optical waveguide and lens to be caused by a temperature change and to prevent a light coupling efficiency from being lowered.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based on Japanese Patent Application No. 2001-191428, filed on Jun. 25, 2001, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   A) Field of the Invention 
   The present invention relates to an optical transmission device, and more particularly to an optical transmission device for optically coupling an optical waveguide formed on a substrate to another optical device. 
   B) Description of the Related Art 
   Optical communication is increasing its speed and capacity because of a broadening transmission band and development of wavelength division multiplexing. In order to configure a hardware infrastructure of an optical fiber network in a trunk communications network, optical switches are required for switching optical signals toward destinations. 
   An example of an optical switch is shown in  FIG. 9A . The optical switch includes a plurality of optical splitters  100 , an optical switch module  101 , a plurality of optical multiplexers  110 , and a plurality of optical amplifiers  111 . An optical fiber  120  is connected to each optical splitter  100 . A wavelength division multiplexed optical signal is supplied from the optical fiber  120  to the optical splitter  100 . The optical splitter  100  splits the wavelength division multiplexed optical signal into a plurality of optical signals. Split optical signals are input to the optical switch module  101  at the succeeding stage. 
   The optical switch module  101  has a three-stage structure. Each stage is constituted of a plurality of optical switch substrates. At the first stage, the optical switch substrate is provided for each optical splitter  100  to switch optical signals from optical waveguides of each optical splitter  100 . The optical switch substrate at the second stage switches optical signals from a plurality of optical switch substrates at the first stage. The optical switch substrate at the third stage switches optical signals from a plurality of optical switch substrates at the second stage. 
   The optical multiplexer  110  is provided for each optical switch substrate at the third stage to multiplex the optical signal output from each optical switch substrate at the third stage. The multiplexed optical signal is amplified by the optical amplifier  111 . An optical connector  115  is provided for connection between the optical splitter  100  and optical switch substrate at the first stage of the optical switch module  101 , between the optical switch substrate at each stage of the optical switch module  101  and optical switch substrate at the succeeding stage, and between the optical switch substrate at the third stage and optical multiplexer  110 . 
     FIG. 9B  is a plan view of the optical switch substrate of the optical switch module  101  shown in  FIG. 9A . An XY rectangular coordinate system is defined on the surface of a rectangular substrate  125 , the X- and Y-axes being parallel to the sides of the rectangle. A plurality of input side optical waveguides  130  are disposed along one side parallel to the Y-axis to transmit light along the X-axis direction. A collimator lens  131  and a beam deflection element  132  are disposed on the surface of the substrate  125  in correspondence with each input side optical waveguide  130 . 
   A beam deflection element  134  on the output side is disposed in correspondence with each beam deflection element  132 , with a slab optical waveguide  133  being interposed therebetween. A condenser lens  135  and output side optical waveguide  136  are disposed in correspondence with each beam deflection element  134 . 
   The input side beam deflection element  132  changes the propagation direction of a light beam in the substrate plane. The light beam with a changed propagation direction propagates through the slab optical waveguide  133  and becomes incident upon the output side beam deflection element  134 . The beam deflection element  134  changes the propagation direction of the light beam to make it incident upon the corresponding condenser lens  135 . The condenser lens  135  converges the light beam at the input end of the corresponding output side optical waveguide  136 . 
   By deflecting a light beam to a desired direction by the input side beam deflection element  132 , the optical signal input to the input side optical waveguide  130  can reach a desired output side optical waveguide  136 . An optical signal can be switched by controlling the deflection direction at each time slot of the optical signal. 
   A method of connecting the output side optical waveguide  136  shown in  FIG. 9B  to the input side optical waveguide of, for example, the optical multiplexer  110  shown in  FIG. 9A , is disclosed in JP-A-2000-304966 and JP-A-5-40214. 
   According to the invention disclosed in JP-A-2000-304966, a lens is disposed in correspondence with each output side optical waveguide between the output side and input side optical waveguides. Each lens converges light output and diverged from a corresponding output side optical waveguide at the input end of the corresponding input side optical waveguide. Since the output ends of the output side optical waveguides are disposed in line, the lenses are made of a micro lens array. 
   According to the invention disclosed in JP-A-5-40214, a collimator lens and a condenser lens are disposed in correspondence with each output side optical waveguide between the output side and input side optical waveguides. Light output and diverged from each output side optical waveguide is changed to a parallel light flux by a corresponding collimator lens, and this parallel light flux is converged at the input end of the input side optical waveguide by the condenser lens. These collimator lenses and condenser lenses are also made of micro lens arrays. Since the light beam between the collimator lens and condenser lens is a parallel light flux, a position alignment precision of a space between the collimator lenses and condenser lenses can be relaxed. Since the lenses have a sealing structure, the inside of the optical system can be protected. The influence of attached dusts can be mitigated. 
   JP-A-5-264874 discloses an optical system of converging light radiated from a light source and makes the light incident upon the input end of an optical fiber. By utilizing a thermal expansion of components for mounting optical elements, a change in the focal length of a lens to be caused by a temperature change can be compensated. 
   A position displacement between an optical waveguide and a lens to be caused by a temperature change is required to be suppressed in order to maintain high a coupling efficiency between the output side and input side optical waveguides. A position displacement (along a direction parallel to the propagation direction of a light beam) to be caused by a change in the focal length of a lens to be caused by a temperature change can be compensated by the method of utilizing the thermal expansion of mount components disclosed in JP-A-5-264874. If the optical waveguide is of a single mode, the position precision of 1 μm or smaller is necessary with respect to two directions perpendicular to the propagation direction of a light beam. 
   If lenses are made of a micro lens array, a distance between lenses changes because of thermal expansion of lens material. If the positions of a particular optical waveguide and a particular lens are set at a high precision, the positions of other optical waveguides and lenses are displaced. 
   SUMMARY OF THE INVENTION 
   An object of this invention is to provide an optical transmission device capable of preventing a position displacement between optical waveguides and lenses to be caused by a temperature change and preventing an optical coupling efficiency from being lowered. 
   According to one aspect of the present invention, there is provided an optical transmission device comprising: at least one optical waveguide end structure formed on an underlying surface, said optical waveguide end structure including an optical waveguide for guiding light along a first direction parallel to the underlying surface and a first lens formed on the underlying surface and being continuous with the optical waveguide at one end thereof, said first lens converging light that is radiated from the end of said optical waveguide and diverges along directions parallel to the underlying surface; a second lens for converging light that is transmitted through said first lens and diverges along directions perpendicular to the underlying surface; and a support member for supporting said first and second lenses. 
   According to another aspect of the invention, there is provided an optical transmission device comprising: first and second optical connectors each having an optical waveguide end structure, a second lens and a support member, the optical waveguide end structure being formed on an underlying surface and including an optical waveguide for guiding light along a first direction parallel to the underlying surface and a first lens being formed on the underlying surface and being continuous with the optical waveguide at one end thereof, the first lens converging light that is radiated from the end of the optical waveguide and diverges along directions parallel to the underlying surface, the second lens converging light that is transmitted through the first lens and diverges along directions perpendicular to the underlying surface, and the support member supporting the first and second lenses; and a coupling member for removably coupling said first and second optical connectors so that a light beam propagating in the optical waveguide of said first optical connector and converged by the first and second lenses is converged by the second and first lenses of said second optical connector toward one end of the optical waveguide of said second optical connector. 
   The first and second lenses can converge a light beam radiated and diverging from the optical waveguide. Since the optical waveguide and first lens are formed on the same substrate, both the optical waveguide and first lens can be easily aligned in position. A cylindrical surface lens can be used as the second lens. If the cylindrical surface lens is used, the position alignment along a direction parallel to the generating line of the curved surface is not required to be strict. It is therefore easy to align the positions of the second lens and optical waveguide. A shift in positions of the optical waveguide and lens to be caused by a temperature change can be avoided. 
   In this specification, the term “cylindrical surface lens” is intended to mean a lens having a cylindrical surface such as a circular cylindrical surface, a parabolic cylindrical surface, etc. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are a plan view and a cross sectional view of an optical transmission device according to a first embodiment of the invention. 
       FIG. 2  is a perspective view showing the main part of the optical transmission device of the first embodiment. 
       FIG. 3  is a cross sectional view of a positioning member for positioning optical waveguides and an external cylindrical surface lens. 
       FIG. 4  is a perspective view showing the main part of an optical transmission device according to a second embodiment. 
       FIGS. 5A and 5B  are schematic diagrams showing the main part of an optical transmission device according to a third embodiment. 
       FIG. 6  is a schematic diagram showing the main part of an optical transmission device according to a modification of the third embodiment. 
       FIG. 7  is a schematic diagram showing the main part of an optical transmission device according to a fourth embodiment. 
       FIGS. 8A and 8B  are perspective views of connectors and sleeves. 
       FIG. 9A  is a schematic diagram showing an optical switch, and  FIG. 9B  is a plan view of an optical switch substrate. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to  FIGS. 1A and 1B  and  FIG. 2 , the first embodiment of the invention will be described.  FIG. 1A  is a plan view of an optical transmission device of the first embodiment, and  FIG. 1B  is a cross sectional view taken along one-dot chain line B 1 —B 1  shown in  FIG. 1A .  FIG. 2  is a perspective view partially broken of the main part of the optical transmission device of the first embodiment. 
   As shown in  FIG. 1A , the optical transmission device of the first embodiment includes an optical waveguide substrate  10 , an external cylindrical surface lens  25  and a connector  28  respectively on the transmission side, an optical waveguide substrate  30 , an external cylindrical surface lens  35  and a connector  38  respectively on the reception side, and a sleeve  40 . Consider an XYZ rectangular coordinate system having as the ZY plane the surfaces of the transmission side optical waveguide substrate  10  and reception side optical waveguide substrate  30 , as the X-axis the propagation direction of an optical signal, and as the Z-axis the normal direction of the substrate. 
   The transmission side optical waveguide substrate  10  is made of silicon, glass or the like. A plurality of optical waveguides  11  are formed on the surface of the transmission side optical waveguide substrate  10 . Although only two optical waveguides are shown in  FIG. 1A , more optical waveguides are usually disposed. Each optical waveguide  11  propagates light along a direction parallel to the X-axis, and the output end of the optical waveguide  11  is disposed in parallel to the Y-axis. An internal cylindrical surface lens  12  is formed on the substrate surface continuously with the output end of each optical waveguide  11 . All the internal cylindrical surface lenses  12  have the same shape so that by moving in translation one internal cylindrical surface lens  12  along a direction parallel to the Y-axis, it can be superposed upon another internal cylindrical surface lens  12 . 
   As shown in  FIG. 1B  and  FIG. 2 , the optical waveguide  11  and internal cylindrical surface lens  12  each have a three-layer structure of a lower clad  13 , a core  14  and an upper clad  15 . The core  14  has a refractive index larger than those of the upper and lower clads  15  and  13 . This three-layer structure is formed by sequentially coating photopolymer on the surface of the substrate  10  and patterning photopolymer layers by photolithography techniques. The width of the optical waveguide  11  and the thickness of the core  14  is about 5 to 10 μm. 
   Each internal cylindrical surface lens  12  has a curved surface having a straight line parallel to the Z-axis as its generating line. The internal cylindrical surface lens  12  converges in the XY plane a light beam  16  radiated and diverging from the output end of the optical waveguide  11 . The curved surface of the internal cylindrical surface lens  12  is optically designed in such a manner that the light beam transmitted through the lens is changed to approximately a parallel light flux in the XY plane. Therefore, a light beam  17  transmitted through the internal cylindrical surface lens  12  is changed to approximately a parallel light flux in the XY plane. 
   The internal cylindrical surface lens  12  has a three-layer structure similar to the optical waveguide  11 . A light beam is therefore confined in the core  13  between the output end of the optical waveguide  11  and output end of the internal cylindrical surface lens  12 , and will not diverge in the ZX plane. The light beam  17  radiated from the output end of the internal cylindrical surface lens  12  diverges along the directions in the ZX plane as shown in  FIG. 1B . 
   The light beam  17  transmitted through the internal cylindrical surface lens  12  becomes incident upon the external cylindrical surface lens  25 . The relative position of the external cylindrical surface lens  25  to the optical waveguide substrate  10  is fixed by the connector  28 . The external cylindrical surface lens  25  has a curved surface having a straight line parallel to the Y-axis as its generating line so that the light beam  17  is converted along the direction in the ZX plane. The curved surface of the external cylindrical surface lens  25  is optically designed in such as manner that the light beam transmitted through the lens is changed to approximately a parallel light flux along the direction in the ZX plane. A light beam  18  transmitted through the external cylindrical surface lens  25  is changed to a parallel light flux along the direction in both the XY and ZX planes. 
   The diameter of the light beam  18  is set preferably to about 300 to 400 μm. A diverging angle of a light beam radiated from the output end of the optical waveguide  11  has a numerical aperture (NA) of about 0.1. It is therefore preferable to set the focal length of the external cylindrical surface lens  25  to about 2 mm. A radius R of curvature of the external cylindrical surface lens  25  is about 1 mm because R=f/(n−1) where f is the focal length and n is the refractive index. The external cylindrical surface lens  25  may be formed by a plurality of lenses. However, it is preferable to form the external cylindrical surface lens by a single lens through curved surface design with aberration correction. A single lens simplifies the structure and reduces the surface reflection area. 
   The structures of the optical waveguide substrate  30 , internal cylindrical surface lens  32 , external cylindrical surface lens  35  and connector  38  respectively on the reception side are similar to those of the optical waveguide substrate  10 , internal cylindrical surface lens  12 , external cylindrical surface lens  25  and connector  28  respectively on the transmission side. The transmission side connector  28  and reception side connector  38  are inserted into the sleeve  40  so that the transmission side external cylindrical surface lens  25  and reception side external cylindrical surface lens  35  face each other, and that on the outer sides thereof, the transmission side internal cylindrical surface lens  12  and reception side internal cylindrical surface lens  32  face each other. 
   The reception side external cylindrical surface lens  35  converges the light beam  18  changed to the parallel light flux by the transmission side external cylindrical surface lens  25 , along the direction in the ZX plane, and makes the light beam incident upon the internal cylindrical surface lens  32 . The internal cylindrical surface lens  32  converges the light beam along the direction in the XY plane and makes the light beam incident upon the input end of the optical waveguide  31 . 
   According to the first embodiment, the optical waveguide  11  and internal cylindrical surface lens  12  on the transmission side are formed at the same time on the same substrate. Therefore, the position alignment of both the optical waveguide and internal cylindrical surface lens can be set correctly and the position displacement to be caused by a temperature change can be avoided. 
   Further, since the generating line of the curved surface of the external cylindrical surface lens  25  is parallel to the Y-axis, it is not necessary to strictly perform the position alignment between the internal cylindrical surface lens  12  and external cylindrical surface lens  25  in the Y-axis direction. Even if the external cylindrical surface lens  25  is expanded or contracted by a temperature change, the position displacement will not occur. 
   Next, with reference to  FIG. 3 , an example of the structure of a positioning member for positioning the substrate  10  and external cylindrical surface lens  25  shown in  FIGS. 1A and 1B  and  FIG. 2  will be described. 
     FIG. 3  is a cross sectional view of the substrate  10 , external cylindrical surface lens  25  and positioning member  41 . The positioning member  41  defines a first reference plane  41   a  in contact with the upper surface of the upper clad  15 ; a second reference plane  41   b  in contact with the side plane of the external cylindrical surface lens  25 ; a third reference plane  41   c  in contact with the edge of the substrate  10 ; and a fourth reference plane  41   d  in contact with the plane of the external cylindrical surface lens  25  on the side of the internal cylindrical surface lens  12 . A through hole  41   e  is formed through the positioning member in an area between the internal cylindrical surface lens  12  and external cylindrical surface lens  25  to form an optical path of a light beam. 
   Since the upper surface of the upper clad  15  contacts the first reference plane  41   a  and the side plane of the external cylindrical surface lens  25  contacts the second reference plane  41   b , the relative positions of the upper clad  15  and external cylindrical surface lens  25  in the Z-axis direction can be determined correctly. Although it is difficult to set a precision of the thickness of the substrate  10  in the sub-micron order or finer, it is relatively easy to set a precision of the thickness of the upper clad  15  in the sub-micron order or finer. By using the upper surface of the upper clad  15  as the positioning reference, a positioning precision of the core  14  and external cylindrical surface lens  25  in the Z-axis direction can be improved. 
   Since the edge of the substrate  10  contacts the third reference plane  41   c  and the external cylindrical surface lens  25  contacts the fourth reference plane  41   d , the internal cylindrical surface lens  12  and external cylindrical surface lens  25  can be aligned in position in the X-axis direction. 
   Next, with reference to  FIG. 4 , an optical transmission device according to the second embodiment of the invention will be described. 
     FIG. 4  is a perspective view partially broken of the main part of the optical transmission device of the second embodiment. Similar to the first embodiment shown in  FIG. 2 , an optical waveguide  11  and an internal cylindrical surface lens  12  are formed on a substrate  10 , each having a lamination of a lower clad  13 , a core  14  and an upper clad  15 . In the first embodiment, the plane of the internal cylindrical surface lens  12  on the output side is exposed. In the second embodiment, the plane of the internal cylindrical surface lens  12  is covered with an optical waveguide layer  20 . 
   The optical waveguide layer  20  has a three-layer structure of a lower clad  21 , a core  22  and an upper clad  23  stacked in this order from the substrate  10  side. The lower clad  21 , core  22  and upper clad  23  are in contact with the lower clad  13 , core  14  and upper clad  15  of the internal cylindrical surface lens  12 . The refractive index of the core  22  partially constituting the optical waveguide layer  20  is smaller than that of the core  14  of the internal cylindrical surface lens  12 . The refractive indices of the upper and lower clads  23  and  21  of the optical waveguide layer  20  are smaller than those of the upper and lower clads  15  and  13  of the internal cylindrical surface lens  12 . The end plane  22 A of the optical waveguide layer  20  on the external cylindrical surface lens  25  is perpendicular to the X-axis. 
   In the first embodiment shown in  FIG. 2 , the light beam transmitted through the internal cylindrical surface lens  12  diverges along the directions in the ZX plane. The curved surface of the internal cylindrical surface lens  12  is a convex curve directing toward the external cylindrical surface lens  25 . Therefore, the X-coordinate values of radiation points of the light beam diverging along the directions in the ZX plane are not the same but different. This different X-coordinate values result in aberration at the time of collimation by the external cylindrical surface lens  25 . 
   In the second embodiment shown in  FIG. 4 , the light beam transmitted through the internal cylindrical surface lens  12  becomes incident upon the optical waveguide layer  20 . Since the light beam is confined in the core  22  of the optical waveguide layer  20 , the light beam will not diverge along the directions in the ZX plane in the optical waveguide layer  20 , and starts diverging at the end plane  22 A on the output side. Since the end plane  22 A on the output side is perpendicular to the X-axis, aberration at the time of collimation by the external cylindrical surface lens  25  can be reduced. 
   Next, the internal cylindrical surface lens  12  and optical waveguide layer  20  of the optical transmission device of the second embodiment will be described. Photopolymer is coated on the surface of the substrate and patterned to form the lower clad  13  of the optical waveguide  11  and internal cylindrical surface lens  12 . Photopolymer is further coated on the substrate and patterned to form the lower clad  21  of the optical waveguide layer  20 . Since a swell is generally formed near at the junction between the lower clads  13  and  21 , the surface is planarized by chemical mechanical polishing (CMP). By repeating similar processes, the cores  14  and  22  and upper clads  15  and  23  can be formed. 
   In the second embodiment shown in  FIG. 4 , although the optical waveguide layer  20  is a slab optical waveguide of the three-layer structure, the optical waveguide layer may be made of a single layer. If the optical waveguide layer  20  is made of a single layer, a light beam cannot be confined in the core. However, divergence along the directions in the ZX plane can be suppressed more than if the light beam is radiated directly in the air from the internal cylindrical surface lens  12 . 
   In the second embodiment, although the end plane of the optical waveguide layer  20  on the output side is flat, it is not necessarily required that the end plane on the output side is flat. The configuration of the end plane of the optical waveguide layer  20  on the output side may be designed so that a difference between the longest and shortest lengths of optical paths of a light beam radiating from the end plane of the optical waveguide layer  20  on the output side and reaching the external cylindrical surface lens  25  becomes smaller than a difference between the longest and shortest lengths of optical paths of a light beam radiating from the internal cylindrical surface lens  12  and reaching the external cylindrical surface lens  25 . Also in this case, the reduction effects of aberration at the time of collimation by the external cylindrical surface lens  25  can be expected. 
   Next, with reference to  FIGS. 5A and 5B , an optical transmission device according to the third embodiment of the invention will be described. 
   As shown in  FIG. 5A , an optical waveguide  11  and an internal cylindrical surface lens  12  are formed on the surface of a substrate  10 , and an external cylindrical surface lens  25  is disposed on the optical path of a light beam transmitted through the internal cylindrical surface lens  12 . This configuration is similar to the first embodiment shown in  FIGS. 1A and 1B  and  FIG. 2 . A distance regulating member  45  is disposed between the substrate  10  and external cylindrical surface lens  25 . The distance between the substrate  10  and external cylindrical surface lens  25  is changed when the distance controlling member  45  is thermally expanded. The distance is regulated in such a manner that the plane of the internal cylindrical surface lens  12  on the output side becomes coincident with the focal point of the external cylindrical surface lens  25 . 
   As shown in  FIG. 5B , as the temperature rises, the focal length f of the external cylindrical surface lens  25  is elongated and the distance regulating member  45  is also elongated to increase the distance between the substrate  10  and external cylindrical surface lens  25 . By representing an elongated length of the focal length f by Δf and representing an increased distance between the internal cylindrical surface lens  12  and external cylindrical surface lens  25  by Δg, if Δf−Δg is 0, it is possible to almost perfectly compensate a change in the focal length of the external cylindrical surface lens  25 . If the absolute value of Δf−Δg is smaller than Δf, the compensation effects of a focal length change can be expected. 
   Consider now that the curved surface of the external cylindrical surface lens  25  has a circular cylindrical surface. The elongated length Δf of the focal length when a temperature is raised by ΔT is given by:
 
Δ f=R (1 +αΔT )/( n+ΔnΔT −1)− R /( n− 1)
 
where n is a refractive index of the external cylindrical surface lens  25  at the room temperature, R is the radius of curvature of the circular cylindrical surface, Δn is a refractive index change rate per 1° C., and α is a coefficient of linear expansion. If the external cylindrical surface lens  25  is made of quartz, n=1.445, Δn=1×10 −5 /° C. and α=0.4×10 −6 /°C. If the radius R of curvature is 2 mm and the temperature rise width ΔT is 100° C., then Δf is 0.010 mm.
 
   Under these conditions, the focal length f at the room temperature is about 4.5 mm. If the material of the distance regulating member  45  has a coefficient of linear expansion of 2.2×10 −5 /° C., a change in the focal length at the temperature rise of 100° C. can be almost perfectly compensated. Such material is, for example, aluminum. 
     FIG. 6  is a schematic diagram of an optical transmission device according to a modification of the third embodiment. In the third embodiment shown in  FIGS. 5A and 5B , the distance regulating member is a single discrete member. In this modification, a distance regulating member  45  is made of two members  45 A and  45 B juxtaposed along the X-axis direction. The two members  45 A and  45 B are made of materials having different coefficients of linear expansion. 
   Under the conditions described with the third embodiment, it is possible to find the material having a desired coefficient of linear expansion. There is a case that proper material having a desired coefficient of linear expansion cannot be found. In such a case, as shown in  FIG. 6 , by using two members  45 A and  45 B of the distance regulating member  45 , an effective coefficient of linear expansion can be set near at the desired coefficient. The effective coefficient of linear expansion of the distance regulating member  45  can be given by:
 
 L   A α A /( L   A   +L   B )+ L   B α B /( L   A   +L   B )
 
where L A  is a length of the member  45 A in the X-axis direction, α A  is a coefficient of linear expansion, L B  is a length of the member  45 B in the X-axis direction, α B  is a coefficient of linear expansion.
 
   Next, with reference to  FIG. 7 , an optical transmission device according to the fourth embodiment of the invention will be described. 
     FIG. 7  is a schematic diagram showing the optical transmission device of the fourth embodiment. The structures of an optical waveguide substrate  10  and an external cylindrical surface lens  25  respectively on the transmission side and the structures of an optical waveguide substrate  30  and an external cylindrical surface lens  35  respectively on the reception side are similar to those of the optical transmission device of the first embodiment shown in  FIGS. 1A and 1B  and  FIG. 2 . On the transmission side, the distance between the substrate  10  and external cylindrical surface lens  25  is regulated by a distance regulating member  46 , and on the reception side, the distance between the substrate  30  and external cylindrical surface lens  35  is regulated by another distance regulating member  47 . 
   Elongated lengths of the focal lengths of the external cylindrical surface lenses  25  and  35  when a temperature rises are represented by Δf 1  and Δf 2 , and increased distances of the distance regulating members  46  and  47  caused by thermal expansion are represented by Δg 1  and Δg 2 . The materials of the distance regulating members  46  and  47  are selected so that the absolute value of Δf 1 −Δg 1 +Δf 2 −Δg 2  becomes smaller than the absolute value of Δf 1 +Δf 2 . It is therefore possible to mitigate the influence of a shift of the focal points to be caused by a change in the focal lengths of the external cylindrical surface lenses  25  and  35 . 
   In the example shown in  FIG. 7 , Δg 1 &gt;Δf 1  and Δg 2 &lt;Δf 2 . In the third embodiment, the ideal case Δg 1 =Δf 1  on the transmission side and Δg 2 =Δf 2  on the reception side. In the fourth embodiment, the influence of a temperature change is mitigated as the total of the transmission and reception sides. In this embodiment, the light beam between the external cylindrical surface lenses  25  and  35  is not a parallel light flux in the strict sense. However, the influence of not a parallel light flux is expected to be small. 
   If the connector of the optical transmission device satisfying the condition of Δg 1 &gt;Δf 1  is a male connector and the connector of the optical transmission device satisfying the condition of Δg 2 &lt;Δf 2  is a female connector, these connectors can be mounted on the optical transmission device without checking the coefficients of linear expansion of the distance regulating members. 
     FIG. 8A  is a perspective view of a transmission side connector  28 , a reception side connector  38  and a sleeve  40  for coupling both the connectors together. An optical waveguide substrate  10  with the transmission side connector  28  and an optical waveguide substrate  30  with the transmission side connector  38  are disposed along the same plane. This connection configuration is utilized for the connection between the optical splitter  100  shown in  FIG. 9A  and the first stage of the optical switch module  101  and for the connection between the third stage of the optical switch module  101  and the optical multiplexer  110 . 
     FIG. 8B  shows another structure of a sleeve. Slots formed on one side of the sleeve  50  are perpendicular to slots formed on the opposite side. A transmission side connector  51  is inserted into a slot on one side, and a reception side connector  52  is inserted into a slot on the opposite side. As viewed in parallel to the propagation direction of a light beam, the direction of output ends of transmission side optical waveguides are perpendicular to the direction of input ends of reception side optical waveguides. This connection configuration is utilized for the connection between the first and second stages of the optical switch module  101  shown in  FIG. 9A  and for the connection between the second and third stages. 
   The optical transmission devices of the first to fourth embodiments are applicable to both the connection configurations shown in  FIGS. 8A and 8B . 
   The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. It is apparent that various modifications, improvements, combinations, and the like can be made by those skilled in the art.