Study Of Nonreciprocal Devices Based On Multimode Interference

Magneto-optic devices, based on nonreciprocal characteristic, play an important role in optical communication networks. Such as mageneto-optic isolators, which are widely used to protect the semiconductor lasers from the unwanted reflected light.Three main principles have been utilized to fabricate magneto-optical isolators and circulators. The first is the Nonreciprocal Mode Conversion. In these stucctrues, the fundamental TM mode is converted to a deep TE radiation mode. But phase matching should be adjusted precisely in this structure. The sencond useful effect is the nonreciprocal loss (NRL), as the optical losses differ between the forward and backward propagation. However, the method to relax the fabrication is still an open issue. Of the most interest is nonreciprocal phase shift (NPS), by which constructive interference in the forward propagation direction and destructive interference in the backward direction can be achieved simultaneously. NPS Multimode interference (MMI) devices have gained widespread application due to their excellent properties in functionality, size and fabrication tolerance. O.Zhuromskyy et al has analyzed MMI isolators for TE modes with magneto-domain structures for the first time. J.S.Yang et al has experimentally fabricated the first MMI isolator on InP substrate, although the isolation is only 2.9dB.In this paper, MMI isolators, based on nonreciprocal phase shift, with asymmetric multimode structures for TM modes, are present. Using the structure Air/Ce:YIG/SiO2 on Si substrate, it is integrate-able with other photonic devices. Equivalent BPM simulation shows that, the optical loss is about 2.4dB and the isolation is as high as 27dB. Polarization independent MMI isolators are also analyzed in this paper. Although the NPS of magneto-domain structure is greater than conventional structure, it is difficult to be fabricated. With a high refractive index material at the side of the magneto-optical waveguide, the same NPS for both TE modes and TM modes may be achieved. So polarization independent isolators can be designed. Based on magneto-optical nonreciprocal characteristics, a "nonreciprocal waveguide splitter" is proposed (Chinese national invention patent, No.200610050282 ), which can simultaneity chieve both 1 ×2 splitter in the forward direction and 1 × 1 or 2 × 1 combinationer in the backward direction. In addition, the nonreciprocal characteristics of YIG material is tested, results shows that, the material is nearly independent with temperature and wavelength, and the transmission index in optical communication band is above 60%; Finally, the process of waveguide on magneto-optic material is explored.