Abstract:
An optical waveguide comprising cladding  1  and core segment  20  buried in cladding  1  and serving as a waveguide, wherein a combination of glass material constituting the core segment  20  and another glass material constituting the cladding  1  is so selected that an absolute value of difference in coefficient of thermal expansion between these materials (α1−α2) is within a range of 0 and 9×10 −7 /° C., where α1 denotes a coefficient of thermal expansion of the former material and α2 denotes that of the latter material. Since this makes possible to bond directly the glass materials having different refraction factors and different coefficients of thermal expansion, and to produce the optical waveguide at even a lower temperature as compared to the prior art method as an upper cladding layer is formed with the sputtering method, it realizes reduction in number of processes and time of manufacture, thereby providing the optical waveguide of low transmission loss at low cost, as well as a method of manufacturing the same.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an optical component, and in particular, it relates to an optical waveguide and a method of manufacturing the same.  
         BACKGROUND OF THE INVENTION  
         [0002]    In optical components of a waveguide system such as optical splitters, optical combiners, optical dividers, optical multiplexers and the like, there hitherto are numerous methods of forming optical waveguides on a substrate, which can be categorized generally into two groups that are deposition method and ion exchange method.  
           [0003]    Deposition method is the method of forming an SiO 2  film on a substrate made of silicon and the like, and it includes the flame hyrolysis deposition method, the plasma assisted chemical vapor deposition method (P-CVD method), the molecular beam epitaxy method (MBE method), and the like, to be more specific.  
           [0004]    Referring now to FIG. 3, description is provided hereinafter of a method of manufacturing an optical waveguide using the flame hyrolysis deposition method representing those of the above deposition methods. After fine-grained silicon dioxide (SiO 2 ) added with germanium dioxide (GeO 2 ), or fine-grained silicon dioxide (SiO 2 ) added with titanium oxide (TiO 2 ) is deposited into a thickness of several μm on quartz substrate  7 , it is sintered and transparentized to form core layer  8 , as shown in FIG. 3( a ). Next, the core layer  8  is patterned into a predetermined shape by using the photolithography and dry etching techniques to form core segments  81 , as shown in FIG. 3( b ). Following the above, silicon dioxide (SiO 2 ) added with phosphorus pentaoxide (P 2 O 5 ) and boric oxide (B 2 O 3 ) is deposited over the core segments  81  and the quartz substrate  7  again by the flame hyrolysis deposition method, and it is transparentized by sintering to form upper cladding layer  9 , as shown in FIG. 3( c ), to thus complete the optical waveguide.  
           [0005]    On the other hand, described next is a manufacturing method using the ion exchange method. In this method, a borosilicate glass substrate is used, and a waveguide pattern is formed on this substrate by using a metal mask. This substrate is then immersed in molten salt containing dopant, to let the dopant diffuse into the glass within the waveguide pattern by taking advantage of the ion-exchange phenomenon of the dopant with the glass components, and to form a core layer in a surface layer of the glass substrate. Because the ion exchange phenomenon does not occur in an area covered by the metal mask, the core layer is produced only in an exposed area into a shape identical to that of the mask pattern.  
           [0006]    In addition, the glass substrate is immersed in another molten salt containing only components other than the aforesaid dopant among those present in the glass substrate, and an electric field is applied to the substrate to cause the dopant in the core layer formed in the surface layer to migrate toward inside of the glass substrate, to form core segments of a predetermined shape in the glass substrate. These core segments are parts having a high refraction factor so as to become optical waveguides. After removal of the metal mask, the surface layer of the glass substrate is clad with a material of low refraction factor in a manner to cover the core segments, which serve the optical waveguides, into such a configuration that the core segments are buried.  
           [0007]    However, there exist the following problems in the deposition method and the ion exchange method described above.  
           [0008]    Although the optical waveguide formed by the deposition method shows the smallest transmission loss, it has a problem in productivity because it requires many processes to make the optical waveguide as it must go through the deposition and sintering processes for the core segments and the cladding layer, and the patterning process using the photolithography method, each of which requires a long duration of processing time. For instance, the core layer requires 1.5 hours for deposition and 10 hours for sintering.  
           [0009]    On the other hand, although the optical waveguide can be formed in a comparatively short time with the ion exchange method, it produces uneven distribution of the dopant in a direction of thickness of the glass substrate because the core layer is formed by ion exchange. This consequently produces uneven variation of refraction factor in the direction of thickness and increases the transmission loss, thereby giving rise to a problem.  
         SUMMERY OF THE INVENTION  
         [0010]    The present invention addresses the aforesaid problems of the prior art, and it is an object of the invention to provide an optical waveguide of low transmission loss, and a method of manufacturing the optical waveguide with low cost by reducing a number of processes as well as time required for manufacturing the same.  
           [0011]    To achieve the above object, the present invention includes structures as described hereinafter.  
           [0012]    An optical waveguide of this invention comprises a cladding and a core segment buried in the cladding and serving as a waveguide, wherein a glass material constituting the core segment and another glass material constituting the cladding are so combined that an absolute value of difference in coefficient of thermal expansion between these materials (α1−α2) is within a range of 0 and 9×10 −7 /° C., when α1 denotes a coefficient of thermal expansion of the former material and α2 denotes that of the latter material.  
           [0013]    Since this makes possible to bond directly between the core segment and the lower cladding layer by using glass materials having different refraction factors and different coefficients of thermal expansion, it realizes good optical characteristic at their interface and increases bonding strength. In addition, the optical waveguide can be produced at even a lower temperature as compared to the prior art method when the upper cladding layer is formed with the sputtering method, thereby realizing not only improvement of flexibility in selecting the materials but also reduction of the cost.  
           [0014]    Furthermore, the optical waveguide of this invention comprises a cladding and a core segment buried in the cladding and serving as a waveguide, wherein a glass material constituting the core segment and another glass material constituting the cladding are so combined that an absolute value of difference in coefficient of thermal expansion between these materials (α1−α2) is within a range of 0 and 9×10 −7 /° C., and a yield point of the glass material constituting the core segment is higher than that of the glass material constituting the cladding by 70° C. or greater, when α1 and At1 denote a coefficient of thermal expansion and the yield point of the former material, and α2 and At2 denote those of the latter material.  
           [0015]    Because this makes possible not only to form the upper cladding layer by the sputtering method, but also to adopt an alternative method in which a glass plate constituting the cladding layer is integrated into one body with a lower cladding layer by hot pressing in a manner to bury the core segment into the cladding layer, thereby realizing a cost reduction by way of reducing a number and time of the manufacturing processes.  
           [0016]    Also, the optical waveguide of this invention has a structure, in which the glass material constituting the core segment comprises borosilicate crown glass having a composition of silicon dioxide (SiO 2 ), boric oxide (B 2 O 3 ), alkaline metal oxide (R 2 O) and diatomic metal oxide (LO) (R: alkaline metal and L: diatomic metal), and the glass material constituting the cladding comprises fluorine crown glass having a composition of silicon dioxide (SiO 2 ), boric oxide (B 2 O 3 ) and fluorine compound. Because of this structure, the optical waveguide has an excellent transmission loss factor, and it can be made with low cost.  
           [0017]    Furthermore, the optical waveguide of this invention has another structure, in which glass materials constituting both the core segment and the cladding comprise fluorine crown glass having a composition of silicon dioxide (SiO 2 ), boric oxide (B 2 O 3 ) and fluorine compound, and the materials are so combined that a refraction factor of the core segment is greater than that of the cladding. Because of this structure, the optical waveguide has an excellent transmission loss factor, and it can be made with low cost.  
           [0018]    Also, a method of manufacturing the optical waveguide according to this invention comprises a step of forming a bonded substrate by pressing and heating a glass plate constituting a lower cladding layer and another glass plate constituting a core layer, which are arranged in a manner that optically polished surfaces of them abut against each other to make direct bonding, a step of finishing a surface of the core layer of the bonded substrate by grinding and/or polishing to obtain a thickness appropriate for a core segment, a step of forming the core segment defining a waveguide by etching the core layer, and a step of forming an upper cladding layer by making a film with sputtering over the lower cladding layer, including the core segment, while burying the core segment at the same time into a cladding comprised of the upper cladding layer and the lower cladding layer. Accordingly, the method can produce the optical waveguide composed of glass materials quite easily while ensuring excellent characteristics.  
           [0019]    Moreover, another method of manufacturing the optical waveguide according to this invention comprises a step of forming a bonded substrate by pressing and heating a glass plate constituting a lower cladding layer and another glass plate constituting a core layer, which are arranged in a manner that optically polished surfaces of them abut against each other to make direct bonding, a step of finishing a surface of the core layer of the bonded substrate by grinding and/or polishing to obtain an appropriate thickness for core segment, a step of forming the core segment defining a waveguide by etching the core layer, and a step of arranging on the core segment an upper cladding layer of glass plate having a yield point lower than that of the core segment, and hot-pressing them by means of heating and pressing at a temperature at least equal to or higher than the yield point of the upper cladding layer, to bond together the upper cladding layer and the lower cladding layer into an integral body and to bury the core segment at the same time into the cladding while retaining an original shape of the core segment. Accordingly, this method can produce the optical waveguide of excellent optical characteristics in a short time with simple manufacturing processes.  
           [0020]    Furthermore, the method of manufacturing the optical waveguide of this invention employs a glass material constituting the core segment and another glass material constituting the cladding, wherein an absolute value of difference in coefficient of thermal expansion between these materials (α1−α2) is within a range of 0 and 9×10 −7 /° C., when α1 denotes a coefficient of thermal expansion of the former material and α2 denotes that of the latter material. The method can produce easily the optical waveguide which is not liable to produce any crack or separation even after the processes of direct bonding, dicing and the like.  
           [0021]    Moreover, the method of manufacturing the optical waveguide of this invention employs a glass material constituting the core segment and another glass material constituting the cladding, wherein an absolute value of difference in coefficient of thermal expansion between these materials (α1−α2) is within a range of 0 and 9×10 −7 /° C., and a yield point of the glass material constituting the core segment is higher than that of the glass material constituting the cladding by 70° C. or greater, when α1 and At1 denote a coefficient of thermal expansion and the yield point of the former material, and α2 and At2 denote those of the latter material. This method can produce optical waveguides with constant precision in shape of the core segments and good productivity in the mass-production even though the upper cladding layers are hot-pressed.  
           [0022]    The method of manufacturing the optical waveguide of this invention includes the step of hot pressing using a temperature higher than the yield point of the glass material constituting the cladding but lower than the yield point of the glass material constituting the core segment. By setting the hot pressing temperature within the above range, this method can reliably bond and integrate the upper cladding layer and the lower cladding layer into one body while controlling deformation of the core segment.  
           [0023]    Moreover, the method of manufacturing the optical waveguide of this invention employs glass material constituting the core segment, which comprises borosilicate crown glass containing silicon dioxide (SiO 2 ), boric oxide (B 2 O 3 ), alkaline metal oxide (R 2 O) and diatomic metal oxide (LO) (R: alkaline metal and L: diatomic metal), and glass material constituting the cladding, which comprises fluorine crown glass containing silicon dioxide (SiO 2 ), boric oxide (B 2 O 3 ) and fluorine compound. This method can make the direct bonding easy so as to increase bonding strength at the interface, and produce highly reliable optical waveguide.  
           [0024]    Also, the method of manufacturing the optical waveguide of this invention employs glass materials constituting both the core segment and the cladding, which comprise fluorine crown glass containing silicon dioxide (SiO 2 ), boric oxide (B 2 O 3 ) and fluorine compound, and a combination of these materials are so selected that a refraction factor of the core segment is greater than that of the cladding. This method can also make the direct bonding easy so as to increase bonding strength at the interface, and produce highly reliable optical waveguide.  
           [0025]    Moreover, the method of manufacturing the optical waveguide of this invention includes the step of direct bonding only after both bonding surfaces of the lower cladding layer and the core layer are polished to 0.1 nm to 1 nm in arithmetic mean surface roughness (Ra) and 0.1 μm to 1 μm in flatness throughout the entire surface areas of the substrate being bonded. This method can thus ensure the direct bonding over the entire surfaces even though it uses a substrate of 3 inches or larger in diameter, for instance, so as to allow manufacturing of optical waveguides with high productivity. In this method, 1 nm or less in the surface roughness Ra can establish reliable interatomic bonding between glass components of the substrate, and 1 □m or less in flatness can make uniform contact of the substrate over the entire surface areas, thereby ensuring reliability and consistency in the direct bonding. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 illustrates processing diagrams showing an optical waveguide and a method of manufacturing the same according to a first exemplary embodiment of the present invention;  
         [0027]    [0027]FIG. 2 illustrates processing diagrams showing another optical waveguide and a method of manufacturing the same according to a second exemplary embodiment of the present invention; and  
         [0028]    [0028]FIG. 3 illustrates processing diagrams showing a method of manufacturing an optical waveguide using the flame deposition hydrolysis method of the prior art. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    Referring now to the accompanying drawings, detailed descriptions will be provided hereinafter of optical waveguides and methods of manufacturing the same according to the preferred embodiments of the present invention.  
         [0030]    (First Exemplary Embodiment)  
         [0031]    [0031]FIG. 1 illustrates processing diagrams for manufacturing an optical waveguide according to the first exemplary embodiment of the present invention. A circular shape fluorine crown glass of 3 inches in diameter, as a substrate constituting lower cladding layer  1   a , and a borosilicate crown glass of the same shape, as another substrate constituting core layer  2  were directly bonded together. FIG. 1( a ) shows an appearance of the bonded state. The direct bonding is made in the following manner. First, one surface of each of the glass substrates is optically polished to such an extent that it becomes 1 nm in arithmetic mean surface roughness (Ra) and 1 μm in flatness. After that, these glass substrates are rinsed to a level of cleanliness that a contact angle of water to these glass substrates is 5 degrees or less. Next, after the polished surfaces of these glass substrates are abutted and pressed against each other, they are heat-treated at 250° C. for one hour to bond the lower cladding layer  1   a  and the core layer  2  directly in the atomic level, and to form integrated bonded substrate  11 . In the direct bonding, when hyaline surfaces of oxide such as glass substrates are normalized in the atomic level and heat-treated while being abutted and pressed against each other, there is produced bonding of atoms of the glass substrate components via oxygen atoms, to gain an interface as strong as a solid bulk. According to the direct bonding as described, integration of materials can be made easily without using any adhesive agent even when they are large in diametral size and different materials containing different compositions.  
         [0032]    Next, a surface of the core layer  2  is ground and polished to a final thickness of 5 to 7 μm, as shown in FIG. 1( b ). The core layer  2  is then coated with photoresist  3  by the spin-coating method, as shown in FIG. 1( c ). Afterwards, the photoresist  3  is patterned by the photolithography method as shown in FIG. 1( d ), and the unnecessary portion of the core layer  2  is removed by dry etching as shown in FIG. 1( e ). This obtains substrate  11   a  provided with core segments  20  of a predetermined shape formed on the lower cladding layer  1   a , when the photoresist  3  is removed thereafter.  
         [0033]    Following the above, upper cladding layer  1   b  is formed by depositing fluorine crown glass with the sputtering method on the surface where the core segments  20  are formed as shown in FIG. 1( f ) using as a target the same fluorine crown glass as the lower cladding layer  1   a . Since the lower cladding layer  1   a  and the upper cladding layer  1   b  are formed of the same material, they are integrated into one body to form cladding  1 , so as to obtain the optical waveguide in which core segments  20  are buried into the cladding  1 . This is shown in FIG. 1( g ).  
         [0034]    Concreately embodied experiment sample is described hereinafter with reference to a comparison sample.  
         [0035]    For a first experiment sample, lower cladding layer  1   a  was formed using fluorine crown glass having a glass yield point of 568° C., a refraction factor of 1.4876, a coefficient of thermal expansion of 95×10 −7 /° C., and composed of SiO 2 , B 2 O 3 , K 2 O and KHF 2 . Also, core layer  2  was formed using borosilicate crown glass having a glass yield point of 625° C., a refraction factor of 1.5164, a coefficient of thermal expansion of 86×10 −7 /° C., and composed of SiO 2 , B 2 O 3 , Na 2 O, K 2 O and BaO. These glass substrates are circular shape, and each has 1 mm in thickness and 3 inches in diameter.  
         [0036]    After each of surfaces to be abutted was optically polished to a surface roughness (Ra) of 1 nm and flatness of 1 μm, and rinsed to such cleanliness that a contact angle of water to the glass substrate becomes 5 degrees or less, the polished surfaces were abutted and pressed upon each other, and subjected to a heat treatment at 250° C. for 1 hour to bond them directly. The borosilicate crown glass constitutong core layer  2  was then ground and polished until it becomes 7 μm thick. Next, photoresist  3  was formed on the borosilicate crown glass, and it was patterned by exposing it to light through a mask pattern placed thereon and by developing it thereafter. Using the patterned photoresist  3  as a mask, the core layer  2  was subjected to the reactive ion etching, to form core segments  20  of 7 μm square in cross-sectional shape. Subsequently, fluorine crown glass of the same composition as the lower cladding layer  1   a  was sputtered to a thickness of 20 μm, which formed upper cladding layer  1   b , and thus completed the optical waveguides. Both the lower cladding layer  1   a  and the upper cladding layer  1   b  were made of the same material having the same optical characteristics, and they composed integrated cladding  1 . The sample produced as above is designated first experiment sample.  
         [0037]    The optical waveguides of this first experiment sample has an absolute value of 9×10 −7 /° C. in difference of coefficient of thermal expansion between the core segments  20  and the cladding  1  (α1−α2). However, the sample have a substantially great bonding strength in the directly bonded interface between the core segments  20  and the cladding  1 , that it did not show any sign of separation at all. In addition, since the lower cladding layer  1   a  and the core segments  20  are covered sufficiently with the upper cladding layer  1   b  formed by the sputtering, there were not observed any air bubbles which are liable to be trapped in any of the interfaces between the core segments  20  and the lower cladding layer  1   a , and between the core segments  20  and the upper cladding layer  1   b.    
         [0038]    In addition, it is necessary that a plurality of the optical waveguides produced in the circular shape glass substrate of 3-inch diameter are cut into individual segments, and ends of the core segments  20  are also cut by a dicer or the like to expose their surfaces in order to connect them with optical fibers. If there is a large difference in the coefficient of thermal expansion between the core segments  20  and the cladding  1 , it is likely that a break or a crack can occur in the core segments  20 , cladding  1 , and especially around their interfacial areas during this process. However, such a defect did not occur in the optical waveguides of the first experiment sample.  
         [0039]    As a second experiment sample, circular shape fluorine crown glass substrates having 3 inches in diameter and 1 mm in thickness were used for core layer and lower cladding layer  1   a . However, the fluorine crown glass used for the lower cladding layer  1   a  has a glass yield point of 491° C., a refraction factor of 1.4816, a coefficient of thermal expansion of 95×10 7 /° C., and composed of SiO 2 , B 2 O 3 , AlF 3 , K 2 O and Na 2 O. Also, the fluorine crown glass used for the core layer  2  has a glass yield point of 568° C., a refraction factor of 1.4876, a coefficient of thermal expansion of 95×10 −7 /° C., and composed of SiO 2 , B 2 O 3 , K 2 O and KF. The optical waveguides were produced using these glass substrates in the same method and conditions as those of the first experiment sample. The sample produced in the manner as described above is designated second experiment sample.  
         [0040]    There was not any warp and separation at the directly bonded interface, nor were any air bubbles trapped around the same interface between the core segments  20  and the cladding  1  in the optical waveguides of the second experiment sample. In addition, there was not any break or crack occured during the process of dicing.  
         [0041]    For the purpose of comparison with the above experiment samples, fluorine crown glass of 3 inches in diameter and 1 mm in thickness was used as lower cladding layer  1   a , and borosilicate crown glass of the same shape as core layer  2 . The fluorine crown glass used for the lower cladding layer  1   a  has a glass yield point of 495° C., a refraction factor of 1.5112, a coefficient of thermal expansion of 100×10 −7 /° C., and composed of SiO 2 , KF, and K 2 O. Also, the borosilicate crown glass used for the core layer  2  has a glass yield point of 625° C., a refraction factor of 1.5164, a coefficient of thermal expansion of 86×10 −7 /° C., and composed of SiO 2 , B 2 O 3 , Na 2 O, K 2 O and BaO. Optical waveguides were produced using these glass substrates in the same method and conditions as those of the first experiment sample. The optical waveguides produced as above is designated first comparison sample.  
         [0042]    In the case of first comparison sample, these optical waveguides showed an absolute value of 14×10 −7 /° C. in difference of coefficient of thermal expansion between the core segments  20  and the cladding  1  (α1−α2).  
         [0043]    During the process of direct bonding for these optical waveguides, there was not observed any warp in the substrate, no separation at the interface between the core layer  2  and the lower cladding layer  1   a , nor were air bubbles trapped around the same interface between the core segments  20  and the cladding  1 , as were the cases of the first experiment sample and the second experiment sample. However, there was a crack occurred during dicing in part of the interfaces between the core segments  20  and cladding  1 .  
         [0044]    Table 1 shows results of these three different samples.  
                                                                       TABLE 1                                   1st   2nd               Experiment   Experiment   1st Comparison           Sample   Sample   Sample                                    Yield point   Cladding   568   491   495       (° C.)   Core   625   568   625           segments       Coefficient   Cladding   95   95   100       of thermal   Core   86   95   86       expansion   segments       (×10 −7 /° C.)       Refraction   Cladding   1.4876   1.4816   1.5112       factor   Core   1.5164   1.4876   1.5164           segments            α1-α2 (×10 =7 /° C.)   9   0   14       Difference in Yield   57   77   130       point (° C.)                  
 
         [0045]    As is obvious from Table 1, the value (α1−α2) of the first experiment sample is 9×10 −7 /° C., the value (α1−α2) of the second experiment sample is 0, and the value (α1−α2) of the first comparison sample is 14×10 −7 /° C. On the other hand, the first comparison sample was the only one in which a crack was observed after it was subjected to the processes of direct bonding, dicing, and the like. In addition, Optical waveguides of like shape were produced by using a variety of materials having different coefficients of thermal expansion according to the same manufacturing method as described above, and they were examined for cracks, separations, and presence of air bubbles. As a result, cracks were found occurred during dicing at least on those optical waveguides that have differences (α1−α2) of the coefficient of thermal expansion equal to or greater than 10×10 −7 /° C. However, no abnormality of crack or the like was observed on those having values of 9×10 −7 /° C. or less.  
         [0046]    According to the above results, it was found that the difference of coefficient of thermal expansion between the core layer  2  and the cladding  1  need to be within a range of 0 and 9×10 −7 /° C. in order to manufacture the optical waveguides by making direct bonding between lower cladding layer  1   a  and core layer  2  and forming upper cladding layer  1   b  by sputtering.  
         [0047]    (Second Exemplary Embodiment)  
         [0048]    [0048]FIG. 2 illustrates processing diagrams for manufacturing an optical waveguide according to the second exemplary embodiment of the present invention. A circular shape fluorine crown glass of 3 inches in the diameter, as a substrate constituting lower cladding layer  4   a , and another fluorine crown glass of the same shape, as a substrate constituting core layer  5  were put together by direct bonding. FIG. 2( a ) shows an appearance of the bonded state. The method of making the direct bonding is same as that described in the first exemplary embodiment, and details are therefore skipped. Bonding directly the core layer  5  and the lower cladding layer  4   a  obtains bonded substrate  21  in which the core layer  5  and the lower cladding layer  4   a  are integrated into one body.  
         [0049]    Next, a surface of the core layer  5  is ground and polished until a thickness of the core layer  5  becomes 5 to 7 μm, as shown in FIG. 2( b ). The core layer  5  is then coated with photoresist  6  by the spin-coating method, as shown in FIG. 2( c ). Afterwards, the photoresist  6  is patterned by the photolithography method as shown in FIG. 2( d ), and the unnecessary portion of the core layer  5  is removed by dry etching. This is shown in FIG. 2( e ). Removal of the photoresist  6  now obtains substrate  21   a  provided with core segments  50  formed thereon. This is shown in FIG. 2( f ).  
         [0050]    Prepared thereafter is upper cladding layer  4   b  made of fluorine crown glass, which is same material as the lower cladding layer  4   a . After the upper cladding layer  4   b  is placed on top of the core segments  50  as shown in FIG. 2( g ), it is hot pressed at a temperature higher than the glass yield point of the upper cladding layer  4   b  by 20 to 30° C. to bury the core segments  50  and to form cladding  4  by integrating the lower cladding layer  4   a  and the upper cladding layer  4   b  into one body.  
         [0051]    For this hot pressing, it is necessary that a combination of the glass materials is so chosen as to satisfy the formula of At1−At2&gt;70, where At1 denotes a yield point of the glass material constituting the core layer  5  and At2 denotes that of the glass material constituting the cladding. With this combination, it is possible to set a range of the heating temperature that can avoid the core segments  50  from warping during the hot pressing, and facilitate integration of the upper cladding layer and the lower cladding layer.  
         [0052]    Concreately embodied experiment samples are described hereinafter with reference to comparison samples.  
         [0053]    For a third experiment sample, fluorine crown glasses of 3 inches in diameter and 1 mm in thichmess were used as the core layer  5  and the lower cladding layer  4   a , after one side surface of each of them was optically polished and rinsed. However, the fluorine crown glass used for the lower cladding layer  4   a  has a yield point (At2) of 491° C., a refraction factor of 1.4816, a coefficient of thermal expansion (α2) of 95×10 −7 /° C., and composed of SiO 2 , B 2 O 3 , AlF 3 , K 2 O and Na 2 O. Also, the fluorine crown glass used for the core layer  5  has a yield point (At1) of 568° C., a refraction factor of 1.4876, a coefficient of thermal expansion (α1) of 95×10 −7 /° C., and composed of SiO 2 , B 2 O 3 , K 2 O and KF. The polished surface of the lower cladding layer  4   a  and the polished surface of the core layer  5  were abutted and pressed upon each other, and subjected to a heat treatment at 250° C. for 1 hour to bond them directly. Roughness (Ra) and flatness of the surfaces after optically polished, and a contact angle of water to the substrate after rinsing were in the same degrees as those of the first exemplary embodiment.  
         [0054]    The core layer  5  was then ground and polished until the thickness becames 7 μm. Following the above, the core layer  5  was coated with photoresist  6 , and it was exposed to light in a prescribed manner to pattern-form the photoresist  6 . Using the patterned photoresist  6  as a mask, the core layer  5  was dry-etched to form core segments  50  of approximately 7 μm square in cross-sectional shape.  
         [0055]    Subsequently, the upper cladding layer  4   b  composed of the same glass material as the lower cladding layer  4   a  was placed on top of the core segments  50 , and it was hot-pressed at 520° C. for 30 seconds. The hot press causes the upper cladding layer  4   b  to deform in a manner to bury the core segments  50  into it until it comes into contact with the lower cladding layer  4   a , and to form the optical waveguides when the upper cladding layer  4   b  and the lower cladding layer  4   a  are integrated to become cladding  4 . The sample thus produced is designated third experiment sample.  
         [0056]    The optical waveguides of the third experiment sample did not cause deformation of the core segments  50 , and retained an excellent precision in shape even after they were subjected to the hot pressing. Moreover, there was not any warp of the substrate or separation at the bonded interface, nor were any air bubbles trapped around the same interface between the core segments  50  and the cladding even after the processes of direct bonding, subsequent hot-pressing, and the like.  
         [0057]    For a fourth experiment sample, fluorine crown glass of 3 inches in diameter and 1 mm in thickness was used as the lower cladding layer  4   a , after one side surface was optically polished and rinsed. Also, borosilicate crown glass of the same shape and rinsed in the same way was used as the core layer  5 . However, the fluorine crown glass used for the lower cladding layer  4 a has a yield point (At2) of 447° C., a refraction factor of 1.4644, a coefficient of thermal expansion (α2) of 93×10 −7 /° C., and composed of SiO 2 , B 2 O 3 , K 2 O and KHF 2 . Also, the borosilicate crown glass used for the core layer  5  has a yield point (At1) of 625° C., refraction factor of 1.5164, a coefficient of thermal expansion (α1) of 86×10 −7 /° C., and composed of SiO 2 , B 2 O 3 , Na 2 O, K 2 O and BaO. Although the optical waveguides were produced with these glass materials, same conditions were used as those of the third experiment sample except that the hot press was given at 470° C. The sample thus produced is designated fourth experiment sample.  
         [0058]    The optical waveguides of the fourth experiment sample did not cause warp and separation, nor were any air bubbles trapped around the interface between the core segments  50  and the cladding  4  even after the processes of direct bonding, subsequent hot-pressing and the like. Moreover, there was not any break, crack or the like due to the dicing. Furthermore, the core segments  50  did not deform by the heat after being subjected to the hot press at 470° C., and retained an excellent precision in shape.  
         [0059]    Next, samples of optical waveguide were produced as second comparison sample for the purpose of comparison with the above experiment samples. Fluorine crown glass of 3 inches in diameter and 1 mm in thichmess was used for the lower cladding layer  4   a , after one side surface was optically polished and rinsed. Also, borosilicate crown glass of the same shape and processed in the same way was used for the core layer  5 . However, the fluorine crown glass used for the lower cladding layer  4   a  has a yield point (At2) of 568° C., a refraction factor of 1.4876, a coefficient of thermal expansion (α2) of 95×10 −7 /° C., and composed of SiO 2 , B 2 O 3 , K 2 O and KHF 2 . Also, the borosilicate crown glass used for the core layer  5  has a yield point (At1) of 625° C., refraction factor of 1.5164, a coefficient of thermal expansion (α1) of 86×10 −7 /° C., and composed of SiO 2 , B 2 O 3 , Na 2 O, K 2 O and BaO. Although the optical waveguides were produced with these glass materials, same conditions were used as those of the third experiment sample except that the hot press was given at 590° C.  
         [0060]    The optical waveguides produced as above did not cause warp of the substrate and separation in the interface, nor was any break, crack or the like due to the dicing even after the manufacturing processes of direct bonding, subsequent hot-pressing and the like. Moreover, the core segments  50  did not deform by the heat after being subjected to the hot press, and retained an excellent precision in shape. However, there were air bubbles trapped around the interfaces between the core segments  50  and the upper cladding layer  4   b  as well as between the lower upper cladding layer  4   a  and the upper cladding layer  4   b , indicating deficiency in integration of the cladding by the hot press.  
         [0061]    Next, as a third comparison sample, fluorine crown glass of 3 inches in diameter and 1 mm in thichmess having one side surface optically polished and rinsed was used as the lower cladding layer  4   a , and borosilicate crown glass of the same shape and processed in the same way was used as the core layer  5 . The fluorine crown glass used for the lower cladding layer  4   a  has a yield point (At2) of 495° C., a refraction factor of 1.5112, coefficient of thermal expansion (α2) of 100×10 −7 /° C., and composed of SiO 2 , KF and K 2 O. Also, the borosilicate crown glass used for the core layer  5  has a yield point (At1) of 625° C., refraction factor of 1.5164, coefficient of thermal expansion (α1) of 86×10 −7 /° C., and composed of SiO 2 , B 2 O 3 , Na 2 O, K 2 O and BaO. Although the optical waveguides were produced with these glass materials, same manufacturing conditions were used as those of the third experiment sample. The sample thus produced is designated third comparison sample.  
         [0062]    The optical waveguide produced as above showed no thermal deformation on the core segments  50  even after they were hot-pressed, and retained an excellent precision in shape. Also, it did not show any warp and separation of the substrate even after the processes of direct bonding, hot-pressing and the like. In addition, no air bubbles were observed around the interfaces between the core segments  50  and the upper cladding layer  4   b , or between the upper cladding layer  4   b  and the lower upper cladding layer  4   a , as have occurred in the second comparison sample due to deficiency in the cladding integration by the hot press. However, there were partial cracks found occured in the interface between the core segments  50  and the cladding  1  during the dicing.  
         [0063]    Table 2 shows results of these four different samples.  
                                                                                   TABLE 2                                   Third   Fourth   Second   Third           Experiment   Experiment   Comparison   Comparison           Sample   Sample   Sample   Sample                                    Yield   Cladding   491   447   568   495       point (° C.)   Core   568   625   625   625           segments       Coefficient   Cladding   95   93   95   100       of thermal   Core segments   95   86   86   86       expansion       (×10 −7 /° C.)       Refraction   Cladding   1.4816   1.4644   1.4876   1.5112       factor   Core   1.4876   1.5164   1.5164   1.5164           segments            α1-α2 (×10 −7 /° C.)   0   7   9   14       Difference in Yield   77   178   57   130       point (° C.)                  
 
         [0064]    As is obvious from Table 2, the third experiment sample and the fourth experiment sample were the only samples that exhibited good precision in shape and did not cause any crack, separation or the like. These samples have a difference (α1−α2) of 7×10 −7 /° C. or less in the coefficient of thermal expansion and a difference of 77° C. or higher in the yield point. On the other hand, the second comparison sample has a difference in the yield point of only 57° C., while a difference in the coefficient of thermal expansion (α1−α2) is 9×10 −7 /° C. Further, although the third comparison sample has a large difference of 130° C. in the yield point, it also has a large difference (α1−α2) of 14×10 −7 /° C. in the coefficient of thermal expansion. These differences in the coefficient of thermal expansion and the yield point were examined of their conditions as to how they affect to preciseness of the shape, cracks and the like.  
         [0065]    In the second comparison sample, although there was not any warp of the substrate and separation in the interface, nor was any crack due to dicing even after the manufacturing processes of direct bonding, hot-pressing and the like, there were air bubbles trapped around the interfaces between the core segments  50  and the upper cladding layer  4   b . Presence of the air bubbles is ascribed to deficiency of the hot press. Therefore, cladding tightness can be inproved and the air bubbles prevented to develop if the hot-press temperature is raised. On the other hand, raising the temperature substantially impairs preciseness of forming shape of the core segments  50 .  
         [0066]    In light of the difference in yield point for possibility of improving the cladding tightness with the fot press while ensuring a level of preciseness in the shape, another examination was made for a condition of air-bubble trapping by using combinations of glass materials having a variety of different yield points. It was found as a result that a difference of 70° C. or higher in temperature of the yield point is needed between the upper cladding layer  4   b  and the core segments  50 . In other words, it was determined that deformation on the core segments  50  can be avoided and excellent bonding tightness is ensured in their interface if there is a difference of 70° C. or higher in the temperature, since the hot-press temperature can be set to a value higher than the yield point of the upper cladding layer  4   b  by 20 to 30° C. but lower than the yield point of the core segments  50 . That is, in this manufacturing method for directly bonding the core layer  5  and the lower cladding layer  4   a , and integrating the upper cladding layer  4   b  into one body with the lower cladding layer  4   a  by the hot press in a manner to bury the core segments  50  therein, it is necessary to arrange an absolute value of difference in the coefficient of thermal expansion (α1−α2) in a range of 0 to 9×10 −7 /° C. and a difference of 70° C. or higher in the yield point.  
         [0067]    In these exemplary embodiments, although the descriptions were provided of the cases in which sodium (Na) and potassium (K) are given as examples of the alkaline metal for the borosilicate crown glass, lithium (Li) may also be used. In addition, although the above descriptions were provided of the case of barium (Ba) as the diatomic metal, it may be any of magnesium (Mg), calcium (Ca) and strontium (Sr). Moreover, although the lower cladding layer and the upper cladding layer were composed by using the same material in these exemplary embodiments, they can be of different compositions so long as they are fluorine crown glasses of the same kind. Furthermore, although the above descriptions were provided of the cases in which fluorine compound of KHF 3  and KF were used as a component of the fluorine crown glasses, this invention is not restrictive to these materials, and they may be substituted with any of MgF 2 , CaF 2 , SrF 2 , BaF 2 , LiF, NaF, and the like.  
       Industrial Applicability  
       [0068]    As has been described, an optical waveguide and a method of manufacturing the same according to the present invention relates to a structure in which glass substrates are bonded directly in the interatomic level, and core segments are buried into a cladding when an upper cladding layer is formed by means of sputtering or hot pressing. The invented method provides advantages of reducing a number of the manufacturing processes, shortening the processing time, and manufacturing easily the optical wave guide having excellent characteristics.