This invention relates to a wye-branching optical circuit having a Y-shaped waveguide, and more particularly to a wye-branching optical circuit having an improved stable branching ratio by suppressing the variation in branching ratios for a range of wavelengths which is caused when the incident light is axially shifted.
A conventional wye-branching optical circuit, shown in FIG. 1, includes an input waveguide 1 and output waveguides 2 and 3, which have equal sectional configurations and refractive indexes. The waveguides 1, 2, and 3 meet at a branching portion 4, thus providing a Y-shaped optical circuit. When only the fundamental mode of a wavelength is excited for the input waveguide 1, the optical power is divided equally at the branching portion 4 and applied to the output waveguides 2 and 3.
In the above described wye-branching optical circuit, if only the fundamental mode of a wavelength is excited for the input waveguide 1, a branching ratio of approximately 50:50 is obtained. This is not a problem.
On the other hand, for instance, when the axis of the incident light is slightly shifted, the higher order modes and leaky modes, in addition to the fundamental mode, are excited in the input waveguide. In this case, at the branching portion 4, the field distributions of the higher order modes and leaky modes depend on the wavelength, and the optical powers of these modes, which are applied to the output waveguides 2 and 3, are changed. That is, the branching ratio deviates from 50:50. Thus, the branching ratio is variable within a certain range of wavelengths. This is a problem.
Even if the axis of the incident light is not shifted, the following problem may result. For example, if the dimensions of the waveguides are not exact or vary, the higher order modes and leaky modes are excited, giving rise to similar problems associated with an axial shift of the incident light, as described above.
This phenomenon occurs particularly when light emerging from a single mode fiber is applied to the input waveguide of the wye-branching optical circuit with its axis shifted from the axis of the input waveguide.
FIG. 2 illustrates an example of the wavelength loss characteristic for light emerging from a single mode fiber which is axially shifted when applied to the input waveguide 1 of the conventional wye-branching optical circuit manufactured according to a two step purely thermal ion-exchange process. The two step purely thermal ion-exchange process is disclosed in Paper No. 369 which was presented by Sugawara, Hashizume and Seki in the nationwide meeting of the Electronic Data Communications Society Semiconductor and Material Field held in 1987, and in "Two-step purely thermal ion-exchange technique for single-mode waveguide devices on glass" [Electron Lett., Vol. 24, No. 20, pp.1258-1259 (1988)] presented by Sugawara, Hashizume and Seki.
Briefly, the two step purely thermal ion-exchange process includes covering a surface of a glass substrate with an ion penetration preventing mask film, forming an opening in the mask film according to a predetermined waveguide pattern, and bringing the mask film covered glass substrate into contact with a fused salt containing univalent cations, so that the ions in the salt are exchanged with the ions in the glass and vice versa. As a result, a high refractive index region substantially semi-circular in cross-section is formed in which the refractive index is gradually decreased towards the inside from the opening of the mask film. Thereafter, the mask film is removed, and the glass substrate is brought into contact with the fused salt including univalent cations which are effective in decreasing the refractive index of glass.
As a result of the two step purely thermal ion-exchange process, the maximum refractive index center of the high refractive index region moves from the surface of the substrate towards the back while the high refractive index region becomes substantially circular in cross-section.
The conventional wye-branching optical circuit thus described is made of linear waveguides for a single mode having a wavelength longer than 1.35 .mu.m, with the axis of the incident light shifted by +2 .mu.m in the direction X from the axis of the input waveguide 1. In this case, as shown in FIG. 2, the branching ratio is constant with wavelengths longer than 1.40 .mu.m. Accordingly, for wavelengths in the range from 1.35 .mu.m to 1.40 .mu.m, the branching ratio changes because of the leaky modes propagating in the input waveguide 1, and for wavelengths shorter than 1.35 .mu.m, the branching ratio changes because of the higher order modes and leaky modes propagating in the input waveguide 1.
Related to the above-described phenomenon, the conventional wye-branching optical circuit is disadvantageous for the following reasons. That is, even though the single mode fiber is set in alignment with the input waveguide so that the branching ratio is 50:50 with one wavelength, the branching ratio varies for a range of wavelengths when the axis of the light applied to the input waveguide is slightly shifted from the axis of the waveguide or when the dimensions of the waveguides are not exact or vary. That is, the branching ratio of the conventional wye-branching optical circuit is not very stable.
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