1. Field of the Invention
The present invention relates to a wavelength router for a separating a light signal having multiple wavelengths to each light signal having its wavelength, and to an optical circuit device (or element) for controlling equiphases of a waveguide light, the device being suitable for use of the wavelength router.
2. Description of the Related Art
A diffraction grating acts as an optical waveguide multiplexing/demultiplexing element or device that has excellent wavelength separation characteristics. The device is also called as a wavelength (de-)multiplexer. Conventionally, there has been proposed a wavelength (de-)multiplexer using diffraction gratings (Reference 1: "Applied Physics Letters Vol. 59, pp. 627-629, August, 1991). The device is a plane waveguide having formed at one end face thereof a diffraction grating structure and having input/output ports provided at the other end face thereof. However, the one-sided input/output port is provided in the device and has n inputs and n outputs (where n is a natural number). Therefore, it is difficult to split or to distribute a plurality of input light signals to a plurality of output ports and therefore the device is difficult to use as a wavelength router.
There has also been proposed an n inputs/n outputs type wavelength router (Reference 2: "Proc. 21 Eur. Conf. on Opt. Comm. '95 Preliminary Draft pp. 195-202, September, 1995). The device has a curved waveguide-array or a plurality of arrayed waveguides as a set of interconnecting waveguides. The arranged waveguides have different lengths, respectively, and are provided between an input-side star coupler and an output-side star coupler. The lights passing through the different arrayed waveguides have different phases, respectively. These lights are incident upon the output-side star coupler and interfere with each other therein. Based on the wavelength dependency of these interference patterns, a light wave is separated to two or more light waves according to wavelengths. The former (de-)multiplexer, using a diffraction grating, utilizes the optical path length difference within the plane waveguide, whereas the latter (de-)multiplexer utilizes the optical path length difference among the respective arrayed waveguides.
However, the wavelength router disclosed in Reference 2 utilizes the difference in optical path lengths of the arrayed waveguides, as mentioned above. Due to this, the wavelength router has a disadvantage in that only a slight error of the waveguide widths of the arrayed waveguide causes a shift in wavelength more easily than the diffraction grating type device.
Meanwhile, there has been further proposed a circuit device for controlling the equiphase-front of waveguided light. The circuits are used as a wavelength router. For example, Reference 3 (Electronics Letters, Vol. 24, pp. 385-386, 1988) discloses the device of this type. The optical circuit device disclosed in Reference 3 comprises an array of curved waveguides having different curvatures and different lengths (see FIG. 1 in Reference 3).
Each of those curved waveguides has a width of 3 mm (line 3, page 386 in Reference 3) and a ridge shape. The device disclosed in Reference 3 controls phases of waveguided light using the difference in the optical path lengths of the curved waveguides. The optical circuit device of this type can be used as various functional devices such as a multiplexer, a demultiplexer and a wavelength filter.
However, in case of the above-stated prior art, if the difference in refractive index between curved waveguides and a substrate, or the widths of the curved waveguides vary, the equivalent refractive index of the curved waveguides vary. In case of Reference 3, for example, the curved waveguides are so narrow, that processing irregularity in waveguide widths is caused and, therefore, a variation in equivalent refractive index of the waveguides tends to occur. The change of the equivalent index of the waveguides causes an error in the phase difference intended to be given by the waveguides. The error is to be referred to as a `phase error` hereinafter. This makes it difficult to control phases as required.
To avoid this problem, there has been proposed a technique disclosed in, for example, Reference 4 or Japanese Unexamined Patent Publication No. 6-194539.
Reference 4 discloses an array of curved waveguides having both ends to which straight waveguides are connected, respectively (see, for example, FIG. 1 in Reference 4). In this technique, as in the case of the above prior art, the arrayed waveguides have different lengths, respectively. Owing to this, it is possible to give predetermined different phases to waveguided lights in the respective waveguides (lines 27-30, column 5, page 4 in Reference 4). However, a fundamental mode light is propagated in the vicinity of the outer edges of the curved waveguides (lines 6-7 from the bottom, column 4, page 3 in Reference 4). The propagation constant is not therefore dependent on the widths of the waveguides (line 5 from the bottom, column 4, page 3 in Reference 4). Therefore, it is possible to prevent a phase error due to a variation in the widths of the curved waveguides.
Even with the technique of Reference 4, a phase error resulting from, for example, the irregularity in the width of the straight waveguides cannot be avoided. In addition, since the curved waveguides have different curvatures (that is, different radiuses), they have different equivalent reflective indexes (which will be described in detail later in comparison with the present invention with reference to FIG. 1). As a result, a phase error eventually occurs.
Therefore, an object of the present invention is to provide an optical circuit device comprising a phase control unit capable of effectively preventing phase errors compared with the conventional techniques.
Another object of the present invention is to provide wavelength router designed not to easily cause a shift in wavelength.