Study Of Tapered MMI Coupler

Integrated optical device has developed rapidly in recent years.The crossing waveguide,as one important part of optical device has a direct impact on the quality of integrated optics.The introduction of rectangular MMI coupler to crossing waveguide will reduce loss and crosstalk caused by optical signal when passing through the crossing waveguide.The loss and crosstalk is still high in traditional rectangular crossing waveguide based on multimode interference.Changing the structure of rectangular MMI coupler appropriately will reduce loss and crosstalk significantly.Therefore,the study of tapered MMI coupler is far-reaching.The introduction of tapered MMI coupler to crossing waveguide is based on the self-imaging of tapered MMI coupler.However,so far the study on the rule of image formation of tapered MMI coupler only involves several special structures such as line-tapered,exponential,quadratic,but not arbitrary structures.On the other hand,the mode coupling will occur when optical signal travels in the tapered MMI coupler,the mode coupling causes some losses.Loss is usually given by simulation while theoretical study on loss caused by the mode coupling is rare.Theoretical derivation is main supplemented by simulation.By employing perturbation theory,we explore mode coupling and loss caused by mode coupling in tapered MMI coupler.By employing guide modepropagation analysis and effective index method,we explore the rule of single self-imaging and general self-imaging of tapered MMI coupler.At last,we design the elliptical crossing waveguide based on multimode interference to demonstrate the superiority of tapered MMI coupler.Main research contents and results are listed as follows:(1)By employing perturbation theory,we analyze mode coupling of tapered MMI coupler,get mode couple coefficient and distance which all the energy is exchanged of trigonometric MMI coupler.Fourier expansion is made for displacement function when we analyze mode coupling of tapered MMI coupler whose structure is arbitrary.At last,correctness of theory is verified by numerical simulation.(2)Rule of single self-imaging of rectangular MMI coupler is analyzed by employing guide mode propagation analysis and effective index method.Then we analyze line-tapered,power function,elliptical,exponential MMI coupler whose structure is more compact and get position of single self-imaging.At last,we summarize the rule of self-imaging and get rule of general self-imaging of tapered MMI coupler.(3)Physical properties of radiation mode is analyzed when propagation constants are imaginary at the same time,emerging conditions is given for this type of radiation mode.(4)Rectangular MMI coupler based on multimode interference by using rule of single self-imaging and self-focusing is designed.The size ofthe device is 13.2 ? m × 13.2 ? m,the loss is about 0.18 dB and the crosstalk is about-44 dB.In order to reduce the loss further,the elliptical MMI coupler is applied to the crossing waveguide by combining imaging theory of the tapered MMI coupler.The size of the device is 13?m×13?m,the loss is reduced to 0.155 dB and the crosstalk is reduced to 46.5dB.