Abstract:
A waveguide type optical branching device having: two input ports; two output ports; optical waveguides extending from the input ports and the output ports respectively; and a coupling portion formed allowing the optical waveguides from the input ports and the output ports to gradually approach each other in a direction toward the output ports. The device is operable such that, in a direction from the input ports to an output end of the coupling portion, when light is inputted to one input port of the two input ports, an even more is predominantly excited, and when light is inputted to the other input port of the two input ports, an odd mode is predominantly excited. The optical waveguides in the coupling portion have a core width that is larger at a middle position in a height direction thereof than at an upper surface of the core.

Description:
[0001]     The present application is based on Japanese patent application No. 2005-302688 filed Oct. 18, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to a waveguide type optical branching device and, particularly, to a waveguide type optical branching device with an X-branching optical circuit for branching light at a specified branching ratio.  
         [0004]     2. Description of the Related Art  
         [0005]     Optical communication system access networks have as their indispensable constituent element an optical branching device (a 2×N coupler) for causing 1.25-1.65 μm wavelength incident light from two input-side optical fibers to branch into plural output-side optical fibers at a constant branching ratio regardless of wavelengths, polarizations, and input ports.  
         [0006]     The 2×N coupler typically comprises a 2-input and 2-output optical branching device (a 2×2 coupler) with a constant branching ratio, and two 1-input and M-output optical branching devices (a 1×M coupler, M=N/2) respectively connected to the output ports of the 2×2 coupler.  
         [0007]     Because the optical properties of the 2×2 coupler are generally poor in comparison to the optical properties of the 1×M coupler, the optical properties of the 2×N coupler are restricted mainly by the optical properties of the 2×2 coupler. Accordingly, in order to realize a high-performance and low-cost 2×N coupler, it is important to realize a 2×2 coupler with a constant branching ratio regardless of wavelengths, polarizations, and input ports.  
         [0008]     Because the 2×2 coupler is one of the most basic constituent elements of the waveguide type optical branching device, if a 2×2 coupler is realized with a constant branching ratio regardless of the conditions such as wavelengths, etc., it is also very useful in applying it to optical communication components other than access networks, or optical waveguide components used in fields other than optical communications  
         [0009]     As techniques for realizing a 2×2 coupler whose branching ratio does not depend on wavelengths, polarizations, and input ports, there are known a fusion coupler using a fused and drawn optical fiber, a Mach-Zehnder interferometer-waveguide type wavelength-independent coupler (MZI-WINC), a waveguide type optical branching device (a waveguide type coupler) using an asymmetrical X-branching optical circuit, and a waveguide type optical branching device using an adiabatic coupler type optical circuit.  
         [0010]     As shown in  FIG. 12 , a waveguide type optical branching device  90  using an adiabatic coupler type optical circuit comprises two input ports  91   a  and  91   b ; two output ports  92   a  and  92   b ; and optical waveguides  93  and  94  for connecting the input ports  91   a  and  91   b  and the output ports  92   a  and  92   b , respectively. The two optical waveguides  93  and  94  gradually approach each other to form a coupling portion  95  comprising the two parallel optical waveguides  93  and  94  close to each other. As shown in  FIG. 13 , the two optical waveguides  93  and  94  comprise a core  97  formed on a substrate  96  and with a rectangular cross section in the light propagation direction, and cladding  98  that covers the core  97 . The G in the figure denotes the length of a gap between two cores  97  in the coupling portion  95 .  
         [0011]     A light signal input from one input port  91   a  of the waveguide type optical branching device  90  is mode-converted to be caused to branch at the optical power ratio of substantially 1 to 1 and output from the two output ports  92   a  and  92   b  (See JP-B-3225819, and Y. Shani et al., “Integrated Optic Adiabatic Devices on Silicon”, IEEE, J. Lightwave Technol., 1991, vol. 27, p.p. 556-566).  
         [0012]     As shown in  FIG. 14 , a waveguide type optical branching device  100  using an asymmetrical X-branching optical circuit comprises, on a substrate not shown, two input ports  101   a  and  101   b ; two output ports  102   a  and  102   b ; and a coupling portion  107  for coupling, in an X shape, optical waveguides  103 ,  104 ,  105  and  106  from the input ports  101   a  and  101   b  and the output ports  102   a  and  102   b  respectively. The two optical waveguides  103  and  104  on the input port  101   a  and  101   b  side have the same core width, while the two optical waveguides  105  and  106  on the input port  102   a  and  102   b  side are formed so that one optical waveguide  105  has a large core width and the other optical waveguide  106  has a small core width.  
         [0013]     In the waveguide type optical branching device  100  of  FIG. 14 , a light signal input from one input port  101   a  is caused to branch at the ratio of substantially 1 to 1 in the coupling portion  107  and output from the two output ports  102   a  and  102   b  (See M. Izutsu, A. Enokihara, T. Sueta, “Optical-waveguide hybrid coupler”, OPTICS LETTERS,  1982 , Vol. 7, No. 11, p.p. 549-551).  
         [0014]     However, there is the problem that the 2×2 coupler using the fusion coupler, or MZI-WINC tends to cause fabrication errors in the optical branching device to be fabricated, resulting from its fabrication method and optical circuit design principle, so that the branching ratio tends to vary according to input wavelengths and input ports.  
         [0015]     For this reason, there arises the problem that the 2×2 coupler using the fusion coupler, or MZI-WINC has difficulty in holding the branching ratio constant for all use wavelengths and all input ports.  
         [0016]     This 2×2 coupler also has difficulty in stable and low-cost fabrication because optical characteristics are easily varied by fabrication errors.  
         [0017]     Further, there is the problem that although in the conventional waveguide type optical branching device using an asymmetrical X-branching optical circuit, or the optical branching device using an adiabatic coupler type optical circuit, asymmetrical branching ratios of e.g., 1:4 or more are verified to be effective, a waveguide type optical branching device with the branching ratio of 1:1 fabricated under actual fabrication conditions causes significant variations in the branching ratio dependent on input wavelengths and input ports, which cannot satisfy required performance of optical communication systems.  
         [0018]     For that reason, in the waveguide type optical branching device  90  explained in  FIGS. 12 and 13 , in order to make the branching ratio constant regardless of input wavelengths and input ports, it is necessary to lengthen the coupling portion  95  (coupling length), or strengthen optical coupling in the coupling portion  95 .  
         [0019]     In the asymmetrical X-branching optical circuit and adiabatic coupler type optical circuit, according to an adiabatic theorem that describes wave propagation in a system in which core cross-sectional shape or structure varies in the light propagation direction, by lengthening the coupling length sufficiently, the branching ratio of the above optical circuit can be constant, but in practice, it is difficult to make the length of the optical circuit more than a certain value because of constraints in fabrication apparatus and fabrication cost. Also, in order to form the long coupling length, increasing the size of the waveguide type optical branching device is not desirable.  
         [0020]     To realize an asymmetrical X-branching optical circuit with a constant branching ratio without lengthening the coupling portion  95 , although it is necessary to narrow a gap G in the coupling portion  95  and thereby strengthen optical coupling between the two optical waveguides, because of the micron-sized gap G in the coupling portion  95 , and therefore constraints in light exposure and etching techniques in conventional fabrication methods, it is difficult to make the gap G in the coupling portion  95  less than a constant value.  
         [0021]     For the above reason, the conventional fabrication methods have difficulty in realizing an asymmetrical optical circuit with a constant branching ratio without increasing practical optical circuit size.  
       SUMMARY OF THE INVENTION  
       [0022]     Accordingly, it is an object of the present invention to provide a waveguide type optical branching device with 2×2 inputs and outputs, capable of obviating the above problems, and having a constant branching ratio regardless of wavelengths, polarizations, and input ports.  
         [0023]     (1) According to one aspect of the invention, a waveguide type optical branching device comprises:  
         [0024]     two input ports;  
         [0025]     two-output ports;  
         [0026]     optical waveguides extending from the input ports and the output ports respectively; and  
         [0027]     a coupling portion formed allowing the optical waveguides from the input ports and the output ports to gradually approach each other in a direction toward the output ports,  
         [0028]     wherein the waveguide type optical branching device is operable such that, in a direction from the input ports to an output end of the coupling portion, when light is inputted to one input port of the two input ports, an even more is predominantly excited, and when light is inputted to the other input port of the two input ports, an odd mode is predominantly excited, and  
         [0029]     the optical waveguides in the coupling portion comprise a core width that is larger at a middle position in a height direction thereof than at an upper surface of the core.  
         [0030]     (2) According to another aspect of the invention, a waveguide type optical branching device comprises:  
         [0031]     two input ports;  
         [0032]     two output ports;  
         [0033]     input waveguides formed connected to the two input ports respectively;  
         [0034]     coupling waveguides formed connected to the input waveguides respectively while allowing the input waveguides to gradually approach each other in a direction toward the output ports;  
         [0035]     output waveguides formed connected to the two output ports respectively;  
         [0036]     branching waveguides formed connected to the output waveguides respectively while allowing the output waveguides to gradually separate each other in the direction toward the output ports; and  
         [0037]     a connection waveguide through which the coupling waveguides and the branching waveguides are connected,  
         [0038]     wherein the waveguide type optical branching device is operable such that, in a direction from the input ports to an output end of the coupling waveguides, when light is inputted to one input port of the two input ports, an even mode is predominantly excited, and when light is inputted to the other input port of the two input ports, an odd mode is predominantly excited; and  
         [0039]     the coupling waveguides comprise a core width that is larger at a middle position in a height direction thereof than at an upper surface of the core.  
         [0040]     In the inventions (1) and (2), the following modifications and changes can be made.  
         [0041]     (i) The coupling waveguides comprise a core width varied asymmetrically, and a spacing therebetween decreased gradually in a direction toward the output ports.  
         [0042]     (ii) The waveguide type optical branching device comprises a branching ratio of 4:1 to 1:4 in a direction from the input ports to the output ports.  
         [0043]     (iii) The core comprises a quartz or an impurity-doped quartz.  
         [0000]     &lt;Advantages of the Invention&gt; 
         [0044]     The present invention exhibits the excellent effect of being able to cause input light to branch at a constant branching ratio regardless of wavelengths, polarizations, and input ports. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0045]     The preferred embodiments according to the invention will be explained below referring to the drawings, wherein:  
         [0046]      FIG. 1  is a plan view showing a first preferred embodiment of a waveguide type optical branching device according to the present invention;  
         [0047]      FIG. 2  is a cross-sectional view along line  2 A- 2 A of  FIG. 1 ;  
         [0048]      FIG. 3  is an explanatory diagram showing the principle of optical coupling in a coupling portion;  
         [0049]      FIG. 4  is an explanatory diagram showing an optical specific electric field distribution in a coupling portion during optical input into one coupling optical waveguide;  
         [0050]      FIG. 5  is an explanatory diagram showing an optical specific electric field distribution in a coupling portion during optical input into the other coupling optical waveguide;  
         [0051]      FIG. 6  is a diagram showing relationships between input and output ports and transmission loss in a conventional waveguide type optical branching device;  
         [0052]      FIG. 7  is a diagram showing relationships between input and output ports and transmission loss in a waveguide type optical branching device of  FIG. 1 ;  
         [0053]      FIG. 8  is a diagram showing the dependency of transmission loss on optical polarization in a waveguide type optical branching device of  FIG. 1 ;  
         [0054]      FIG. 9  is a diagram showing the dependency of transmission loss on wavelength in a waveguide type optical branching device of  FIG. 1 ;  
         [0055]      FIG. 10  is a plan view showing a second preferred embodiment of a waveguide type optical branching device according to the present invention;  
         [0056]      FIG. 11  is a plan view showing a third preferred embodiment of a waveguide type optical branching device according to the present invention;  
         [0057]      FIG. 12  is a plan view showing a conventional waveguide type optical branching device with an adiabatic coupler type optical circuit;  
         [0058]      FIG. 13  is a cross-sectional view along line  13 A- 13 A of  FIG. 12 ; and  
         [0059]      FIG. 14  is a plan view showing a conventional waveguide type optical branching device with an X-branching optical circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0060]      FIG. 1  is a plan view showing a first preferred embodiment of a waveguide type optical branching device according to the present invention.  
         [0061]     As shown in  FIG. 1 , a waveguide type optical branching device  10  includes an optical waveguide comprising, on a substrate  11 , a core and cladding that covers the core. The waveguide type optical branching device  10  has two input ports  12  and  13  and two output ports  14  and  15 , formed in substrate  11  end faces, respectively, opposite each other. The input ports  12  and  13  and the output ports  14  and  15  are respectively used for light inputting from and outputting to outside.  
         [0062]     Connected to the two input ports  12  and  13  are input waveguides  21  and  22  respectively. Connected to the two output ports  14  and  15  are output waveguides  23  and  24  respectively. Connected to the two input waveguides  21  and  22  are coupling waveguides  25  and  26  respectively that gradually approach each other. Connected to the two output waveguides  23  and  24  are branching waveguides  27  and  28  respectively that gradually approach each other. The coupling waveguides  25  and  26  and the branching waveguides  27  and  28  are connected to each other via one connection waveguide  29 .  
         [0063]     As shown in  FIG. 2 , in this embodiment, the substrate  11  is formed of quartz or Si. Formed in the upper surface of the substrate  11  is a lower cladding layer  41  comprising pure or impurity-doped quartz. Formed in the upper surface of the lower cladding layer  41  is a core  42  comprising pure or impurity-doped quartz. Deposited thereover is an upper cladding layer  43  comprising pure or impurity-doped quartz to cover the lower cladding layer  41  and the core  42 . The relative refractive index difference A between the cladding layers and the core is 0.3-0.4%, and the thickness of the core  42  is 7.0-8.0 μm.  
         [0064]     Where the substrate  11  is formed of quartz, it is not necessary to provide the lower cladding layer  41 .  
         [0065]     Returning to  FIG. 1 , in the waveguide type optical branching device  10 , the two input waveguides  21  and  22  serve as an input-side pitch conversion portion  16 ; the coupling waveguides  25  and  26  that approach each other serve as a coupling portion  17 ; the connection waveguide  29  serve as a connection portion  18 ; the branching waveguides  27  and  28  that approach each other serve as a branching portion  19 ; and the two output waveguides  23  and  24  serve as an output-side pitch conversion portion  20 .  
         [0066]     In the input-side pitch conversion portion  16 , the input waveguides  21  and  22  are bent to cause the two input waveguides  21  and  22  to approach each other immediately before optical coupling occurs, and thereby serve to prevent the coupling portion  17  from being extended more than necessary.  
         [0067]     The coupling portion  17  is the most essential in realizing the waveguide type optical branching device  10 . Its two waveguides are formed parallel to have micro-spacing therebetween, and the optical coupling strength between the coupling waveguides  25  and  26  is weak, so that light that propagates through the coupling portion  17  has two super-modes.  
         [0068]     Specifically, as the coupling portion  17 , one coupling waveguide  25  (upper in the figure) is formed to gradually increase its core width towards an output end  33 , while the other coupling waveguide  26  (lower in the figure) is formed to gradually decrease its core width towards the output end  33 , and the distance between both the coupling waveguides  25  and  26  to also gradually decrease towards the output end  33 .  
         [0069]     The coupling portion  17  serves to control amplitude and phase distributions of the electric field component of light, so that the light input from one input end  31  excites only a substantially even symmetrical super-mode (an even mode) at the output end  33  of the coupling portion  17 , while the light input from the other input end  32  excites only a substantially odd symmetrical super-mode (an odd mode) at the output end  33  of the coupling portion  17 .  
         [0070]     For this reason, the coupling portion  17  has structure that satisfies the following two conditions:  
         [0000]     (A) The two coupling waveguides  25  and  26  have different core widths W 1  and W 2  at the output end  33 .  
         [0000]     (B) The distance between the two coupling waveguides  25  and  26  gradually decreases from the input ends  31  and  32  to the output end  33 .  
         [0071]     The connection portion  18  has one connection waveguide  29  that connects the output end  33  of the coupling portion  17  and an input end  34  of the branching portion  19 . The connection waveguide  29  serves to adjust relative phases of unwanted electric field components other than the super-modes that have occurred in the coupling portion  17 , and thereby reduce the dependency of the branching ratio on the input port.  
         [0072]     The connection waveguide  29  is designed so as not to change, as much as possible, the optical electric field distribution in a waveguide cross section perpendicular to the propagation direction from the output end  33  of the coupling portion  17  to the input end  34  of the branching portion  19 . The connection portion  18  may be entirely omitted, to directly connect the output end  33  of the coupling portion  17  and the input end  34  of the branching portion  19 .  
         [0073]     The branching portion  19  and the output-side pitch conversion portion  20  serve to distribute power of the even symmetrical super-mode (the even mode) and the odd symmetrical super-mode (the odd mode) that have passed through the input end  34  of the branching portion  19 , to the output ports  14  and  15  spaced apart with a specified distance, at a constant branching ratio, to output to outside. In the branching portion  19 , the two branching waveguides  27  and  28  are formed in a symmetrical Y-branch shape to have the branching ratio of 1:1, but may be formed in another shape that has an asymmetrical Y-branch, a sinusoidal curved pattern with two tapers, a tapered pattern, a circular arc pattern with an offset, or a linear pattern with two tapers.  
         [0074]     Here, a mechanism of optical coupling in the coupling portion  17  will be explained.  
         [0075]     As shown in  FIG. 3 , realizing an asymmetrical X-branching circuit with a constant branching ratio regardless of wavelengths, polarizations, and input ports requires matching the electric field distribution of input light into the output waveguides  23  and  24 , to a basic (0-order) mode (a in the figure) and a high order (1-order) mode (b in the figure) at the input end  34  of the branching waveguides  27  and  28 , regardless of wavelengths, polarizations, and input ports.  
         [0076]     The shape, equivalent refractive index of the two coupling waveguides  25  and  26  are determined in the following way: As shown in  FIG. 4 , light input from the coupling waveguide  25  in the coupling portion  17  whose core has a large diameter is coupled to the coupling waveguide  26  with a core whose diameter gradually becomes small, so that the electric field distribution at the output end  33  of the coupling portion  17  is matched to the basic mode (a in  FIG. 3 ) at the input end  34  of the branching waveguides  27  and  28 . As shown in  FIG. 5 , light input from the coupling waveguide  26  in the coupling portion  17  whose core has a small diameter is coupled to the coupling waveguide  25  with a core whose diameter gradually becomes large, so that the electric field distribution at the output end  33  of the coupling portion  17  is matched to the high order mode (b in  FIG. 3 ) at the input end  34  of the branching waveguides  27  and  28 .  
         [0077]     The structure of the coupling waveguides  25  and  26  is not limited to the shape shown in  FIG. 1 , but may be such that the light input from the input port  12  excites only a substantially even symmetrical super-mode (an even mode) at the output end  33  of the coupling portion  17 , while the light input from the input port  13  excites only a substantially odd symmetrical super-mode (an odd mode) at the output end  33  of the coupling portion  17 .  
         [0078]     It is assumed that plural optical circuit patterns of the coupling portion  17  can satisfy simultaneously the above-mentioned two conditions (A) and (B). However, it is generally known from experience that the above-mentioned conditions (A) and (B) cannot be satisfied unless the coupling waveguide length (a region where the two coupling waveguides  25  and  26  are close to each other to allow optical power interaction) is sufficiently long, or unless the distance (gap) between the two coupling waveguides  25  and  26  is sufficiently small, regardless of optical circuit pattern shapes.  
         [0079]     In consideration of this, as shown in  FIG. 2 , the waveguide type optical branching device  10  of this embodiment is characterized in that the gap G′ of the middle portion of the waveguide core is smaller than the gap G of the upper surface of the core, specifically, the widths W 1 ′ and W 2 ′ of the middle portion in the height direction of the core are larger than the widths W 1  and W 2  of the upper surface in the height direction of the core.  
         [0080]     In the coupling portion  17  of this waveguide type optical branching device  10 , the two coupling waveguides  25  and  26  have inclined surfaces  44  and  45  on the core side, so that they respectively have the widths W 1 ′ and W 2 ′ in the middle portion of the core larger than the widths W 1  and W 2  of the upper surface of the core. This forms the gap G′ between the core middle portions narrower than the gap G of the core upper surface.  
         [0081]     In the coupling portion  17 , the two coupling waveguides  25  and  26  are formed in a linearly-tapered shape in which the core width is linearly varied in the light propagation direction, or in a curvedly-tapered shape in which the core width is curvedly varied in the light propagation direction. The core width at the output end  33  is such that the core width W 1  of the upper surface of one coupling waveguide  25  is 7.0-9.0 μm, while the core width W 2  of the upper surface of the other coupling waveguide  26  is 2.0-3.0 μm. The length of the connection portion  18  is 1 μm. The branching waveguides  27  and  28  of the branching portion  19  have the same core width to have the branching ratio of 1:1. The length L from the input ends  31  and  32  of the coupling portion  17  to the output end  35  of the branching portion  19  is 8-10 μm.  
         [0082]     Although the waveguide type optical branching device  10  is made by forming an optical circuit pattern with photolithography and etching, the sidewalls  44  and  45  of the core are inclined so that the gap G′ between the core middle portions can thereby be smaller than the gap G of the core upper surface determined by a light exposure accuracy limit. Accordingly, a waveguide type optical branching device can be formed that can realize a narrow gap, i.e., strong optical coupling using a conventional fabrication method, thereby allowing the branching property of an asymmetrical X-branching optical circuit.  
         [0083]     Here, shown in  FIG. 6  is the transmission loss of a waveguide type optical branching device (G=G′) whose coupling waveguides  25  and  26  have vertical core sidewalls, and whose other optical circuit pattern is formed in the same way as that of the waveguide type optical branching device  10  of  FIG. 1 , while shown in  FIG. 7  is the transmission loss of the waveguide type optical branching device  10  of the embodiment of  FIG. 1  (G&gt;G′) whose coupling waveguides  25  and  26  have inclined core sidewalls  44  and  45 . In  FIGS. 6 and 7 , the vertical axis represents the transmission loss, and the horizontal axis device No.A-No.D and input/output port No. For example, shown in  FIG. 6  are results of measuring transmission losses by modifying the combination of the input/output ports and measurement wavelength for the 4 waveguide type optical branching devices A-D fabricated in the same shape and same conditions. The port numbers respectively represent inputs-outputs. For example, the 1-1 represents the transmission loss of light input from input  1  and output from output  1  (see  FIG. 1 ). The characteristic lines  51  and  53  represent the case where the input wavelength is 1.31 μm, and the characteristic lines  52  and  54  represent the case where the input wavelength is 1.55 μm.  
         [0084]     As shown in  FIG. 6 , the waveguide type optical branching devices whose coupling waveguides  25  and  26  have vertical core sidewalls have variations of the order of 1.5 dB in the transmission loss due to input wavelengths and input ports, whereas as shown in  FIG. 7 , the waveguide type optical branching devices  10  of the embodiment whose coupling waveguides  25  and  26  have inclined core sidewalls  44  and  45  can inhibit variations below 0.5 dB in the transmission loss due to input wavelengths and input ports, so that practically adequate characteristics can be obtained.  
         [0085]     The characteristic results of the waveguide type optical branching devices shown  FIGS. 6 and 7  show that because of use of the same fabrication conditions other than the angles of the sidewalls of the coupling waveguides  25  and  26 , by inclining the sidewalls of the core, an asymmetrical X-branching optical circuit can be realized with a constant branching ratio regardless of wavelengths and input ports.  
         [0086]     As shown in  FIG. 8 , in the polarization dependent loss (PDL) of the waveguide type optical branching device of this embodiment, variations due to wavelengths and input ports are inhibited below 0.5 dB, and the dependency of the transmission loss on polarization can also be reduced.  
         [0087]     As shown in  FIG. 9 , variations in the transmission loss at wavelengths of 1.25-1.65 μm are below 0.3 dB, so that there is little dependency of the transmission loss on wavelengths.  
         [0088]     In this manner, by choosing the shape of the sidewalls of the coupling waveguides  25  and  26  to be G-G′&gt;0, the effective gap can be small, to be able to improve the branching ratio property of the asymmetrical X-branch, to realize an ideal 2×2 coupler with little variations in the branching ratio due to wavelengths, polarizations, and input ports.  
         [0089]     Although in this embodiment, only the core sidewalls  44  and  45  of the coupling waveguides  25  and  26  have been formed in the inclined surfaces, core side surfaces of all the optical waveguides on the substrate (input waveguides  21  and  22 , output waveguides  23  and  24 , branching waveguides  27  and  28 , connection waveguide  29 ) may be formed in the inclined surfaces. Forming the core of all the optical waveguides in the inclined surfaces has the merit of reducing the number of fabrication steps such as light exposure, etching, etc.  
         [0090]     Although in this embodiment, the waveguide type optical branching device has the output-side branching ratio of 1:1, it is possible to easily realize a 2×2 coupler with a branching ratio of the order of 1:4-4:1.  
         [0091]     Next, a second preferred embodiment of the present invention will be explained by reference to  FIG. 10 .  
         [0092]     Because basic constituent portions are substantially the same as those of the waveguide type optical branching device  10  of  FIG. 1 , the same constituent portions are denoted by the same characters as in  FIG. 1 , the difference is that a branching portion  71  has two asymmetrical branching waveguides with a branching ratio other than 1:1.  
         [0093]     As shown in  FIG. 10 , in a waveguide type optical branching device  70  of this embodiment, at the input end  34  of the branching portion  71 , the width of one branching waveguide  72  (on the upper side in the figure) is formed to be larger than the width of the other branching waveguide  73  (on the upper side in the figure). The branching waveguide  72  is formed to gradually decrease its core width towards the output waveguide  23 , while the branching waveguide  73  is formed to gradually increase its core width towards the output waveguide  24 . The branching waveguides  72  and  73  have the same core width as that of the output waveguides  23  and  24  respectively so as to be connected thereto.  
         [0094]     As shown in  FIG. 11 , a waveguide type optical branching device  80  of a third preferred embodiment is different in branching portion  81  and output-side pitch conversion portion  82  from the waveguide type optical branching device  10  of  FIG. 1 . In this waveguide type optical branching device  80 , both branching waveguides  83  and  84  have the same core width at the input end  34  of the branching portion  81 , and one branching waveguide  83  (on the upper side in the figure) is formed to have a constant core width towards the output waveguide  23 , while the other branching waveguide  84  is formed to gradually decrease its core width towards the output waveguide  85 , and the output waveguide  85  connected to the branching waveguide  84  is also formed to gradually decrease its core width towards the other output waveguide  23 .  
         [0095]     In the waveguide type optical branching devices  70  and  80  of the second and third embodiments, the waveguides of the branching portions  71  and  81  are significantly arbitrary in shape (core width at input and output ends, taper shape, circuit shape), so that a numerical calculation method such as a beam propagation method is used to appropriately select parameters such as shape, length, relative refractive index difference, etc. and form the waveguides of the branching portions  71  and  81  to thereby be able to cause light to branch at a desired branching ratio.  
         [0096]     The waveguide type optical branching devices  70  and  80  shown in  FIGS. 10 and 11  respectively also have the core middle widths larger than the core upper surface widths in the coupling portion  17 , as in the waveguide type optical branching device  10  of  FIG. 1 , to thereby be able to increase optical coupling strength and improve the branching property of the asymmetrical X-branching optical circuit.  
         [0097]     The effective scope of the present invention is not constrained to the above parameter range. The method of the present invention, which inclines the sidewalls of the core to improve the branching property of the asymmetrical X-branching optical circuit, may be applied to various waveguide materials, relative refractive index differences, core film thicknesses, optical circuit patterns in the coupling portion and in the branching portion.  
         [0098]     The waveguide type optical branching devices  10 ,  70  and  80  of the first-third embodiments may be used as branching devices inside an interferometer. A branching circuit, which has small dependency on wavelengths, polarizations, and input ports, is required for inputs and outputs of an interferometer of each kind, such as Mach-Zehnder, Michelson, Giles Tourmore. For this reason, the waveguide type optical branching device  10  of this embodiment is useful for broadening the band of operating wavelength of the interferometer.  
         [0099]     Particularly, using the waveguide type optical branching devices  10  in the Mach-Zehnder interferometer is useful for broadening the band of a wavelength splitter, a VOA (a variable optical attenuator), a 1×2 optical switch, and a 2×2 optical switch, compared to the prior art.  
         [0100]     The waveguide type optical branching devices  10 ,  70  and  80  may be used as branching devices for optical intensity monitors. They are also useful as broadband optical intensity monitors, such as a power monitor for Raman amplification systems, measurement, fabrication, etc. because of broad operating wavelength bands and small characteristic variations due to fabrication errors.  
         [0101]     The waveguide type optical branching devices  10 ,  70  and  80  may further be applied to a device, which requires broadband X-branching, such as a device for visible light, which requires treating wavelengths in the wide range over 400-700 nm, or a nonlinear optical device, which requires plural wavelength light, such as difference-frequency mixing, second high harmonic generation, four lightwave mixing, parametric amplification, or parametric oscillation.  
         [0102]     Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.