ABSTRACT
A waveguide Y-Branch using single-mode waveguide can easily be designed and fabricated by a proper waveguide pattern with small branching angle. The branching angle must be smaller than the propagation angle of the fundamental mode. If the branching angle is small enough, the branching occurs adiabatically and the branching loss can be reduced to almost zero. However, in the case of the combiner that can be obtained by reversing the input and output ends, the situation is different. A principle 3 dB loss occurs, if the light is incident on one of input ports of the combiner. This is understood by the superposition of in-phase and out-of-phase incidence. Since the power coupled to the out-of-phase component is radiated in the tapered portion of the combiner and the remaining in-phase component is guided into the single mode output port. Therefore, 3 dB loss always occurs.
