1.  Field of the Invention
The present invention relates to a waveguide type optical splitter used for branching light into a plurality of rays of light in a PON system (Passive Optical Network) and the like, for example, and to a waveguide type optical module comprising the same. 
2.  Description of the Related Art
For building a low-cost optical communication system, it is in the process of applying the PON system to the optical communication system.  The PON system makes it possible to achieve an effective usage by splitting the optical fiber using an optical splitter in an access interval.  By applying the PON system to the optical communication system, it is possible to reduce the cost of the optical communication system remarkably. 
The PON system uses an optical splitter module for splitting the light.  A low insertion loss, a low light-dependency, and a high reliability are required for the optical splitter module.  A PLC (Planar Lightwave Circuit) type optical splitter satisfies these requirements and, as a key device for the PON system, tens of thousands of them are manufactured in Japan alone. 
In the PLC type optical splitter, although it is small and has a highly reliable property, there is a birefringence being generated due to a difference between a linear expansion coefficient of an Si (silicon) substrate and that of SiO2 (silica glass).  The birefringence influences the optical property of the optical splitter.  When the waveguide of the optical splitter with the birefringence is branched in Y-shape, the branching ratio becomes dependent on the polarization.  Thus, a polarization dependent loss (PDL) is generated in the branched lights.  The PDL gives an influence to a power margin of the optical communication system so that it is essential to be suppressed. 
The PLC type optical splitter (a first conventional example) will be described in detail by referring to FIG.  1. 
As shown in FIG.  1, a PLC type splitter 80 comprises 1×n (n is an integer of 2 or larger, for example, n=8) of waveguides 82 formed on a substrate 81.  An input optical fiber 84 is connected to an input end 83 of the substrate 81 and a plurality of output optical fibers 86 are connected to an output end 85 of the substrate 81.  A single waveguide 82 is simply branched into a Y-shape and a plurality of divided waveguides 82 are connected in multiple stages, thereby obtaining a plurality of the waveguides 82 in the number consistent with the number of output optical fibers 86. 
However, by employing the multistage Y-branch structure in the PLC type optical splitter 80 with a large birefringence, the PDL increases cumulatively.  Thus, it may not be able to meet customers' needs.  Especially, the number of branches becomes large so that the cumulative PDL cannot be ignored in the multi-branch structure of 1×32, etc.  Further, in the multi-branch PLC type optical splitter 80, the insertion loss is also increased due to the accumulation of excessive branch loss in the Y-branch section. 
In order to achieve an excellent optical property by overcoming the shortcomings of the PLC type optical splitter 80 as described above, it is desirable to employ a so-called star coupler which branches a light signal into a plurality of light signals by a single slab waveguide.  However, in the conventional star coupler, the light power of the light signal in the center becomes larger among the light power of the light signal to be branched and the light power of the light signal becomes smaller from the center portion towards the peripheral portion.  Therefore, although the polarization dependency is not cumulated and an excellent polarization dependency can be obtained in the conventional star coupler since the waveguides therein are not in the multistage structure, it is an issue that the light power varies depending on the positions of the branched light signals.  Japanese Patent No.  2538099 discloses a star coupler for overcoming such shortcomings. 
The star coupler (a second conventional example) of Japanese Patent No.  2538099 will be described in detail by referring to FIG.  2. 
As shown in FIG.  2, in a star coupler 90, disposed and connected in order on a silicon substrate 91 are an input waveguide 92, a fan-shape slab waveguide 93, a number of tapered waveguides 94, and a number of output waveguides 95.  In the star coupler 90, the tapered waveguide 94 is provided respectively to all the output waveguides 95, and an opening width of each tapered waveguide 94 on the fan-shape slab waveguide 93 side is set narrower in the center where the light intensity is strong while it is wider in the peripheral portion where the light intensity is weak so as to make the intensities of the light signals branched by all of the output waveguides 95 uniform (that is, to make the loss uniform). 
As shown in FIG.  2, in the star coupler 90, the tapered waveguide 94 is respectively provided to all the output waveguides 95 and the opening width of each tapered waveguide 94 is widened from the center towards the periphery.  Further, as shown in FIG.  2, the output waveguides 95 positioned in the center portion with the strong light intensity comprises the tapered waveguide 94 like the output waveguide 95 positioned in the peripheral portion with the weak light strength.  In addition, the opening widths of all the tapered waveguides 94 are set wider than the waveguide widths of the output waveguides 95.  Further, the opening widths are widened from the center towards the periphery. 
Therefore, when forming all the output waveguides 95 within the area of the substrate 91, the number of the output waveguides 95 formed on the substrate 91 is limited since there is a limit for narrowing the pitch in between the adjacent output waveguides 95.  Further, when forming the required number of output waveguides 95 on the substrate 91, it is necessary to narrow the opening widths of the tapered waveguides 94 to be in the designated value or smaller.  Thus, improved uniformity of the losses between a plurality of the output waveguides 95 cannot be achieved. 
Actually, with the star coupler which was formed according to the conditions disclosed in Japanese Patent No.  2538099, uniformity of the losses cannot be sufficiently achieved.  In the star coupler 90, as described above, it was difficult to achieve both the high-density output waveguides 95 and uniformity of the loss.