Unitary transformation of general nonoverlapping-image multimode interference couplers with any input and output ports

An explanation of optical unitary transformation is presented for general nonoverlapping-image multimode interference(MMI)couplers with any number of input and output ports.The light transformation in the MMI coupler can be considered as an optical field matrix acting on an input light column vector.We investigate the general phase principle of output light image.The complete proof of nonoverlapping-image MMI coupler’s optical unitarity along with the phase analysis of matrix element is provided.Based on a two-dimensional finite-difference time-domain(2 D-FDTD)simulation,the unitary transformation is obtained for a 4×4 nonoverlapping-image MMI coupler within a deviation of 4×10-2 for orthogonal invariance and 8×10-2 for transvection invariance in the C-band spectral range.A compact 1×4 splitter based on cascaded MMI coupler is proposed,showing a phase deviation less than 5.4°while maintaining a low-loss performance in C-band spectra.

Photon propagation control on laser-written photonic chips enabled by composite waveguides

Femtosecond laser direct writing(Fs LDW)three-dimensional(3D)photonic integrated circuits(PICs)can realize arbitrary arrangement of waveguide arrays and coupling devices.Thus,they are capable of directly constructing arbitrary Hamiltonians and performing specific computing tasks crucial in quantum simulation and computation.However,the propagation constantβis limited to a narrow range in single-mode waveguides by solely changing the processing parameters,which greatly hinders the design of Fs LDW PICs.This study proposes a composite waveguide(CWG)method to increase the range ofβ,where a new single-mode composite waveguide comprises two adjacent circular waveguides.As a result,the photon propagation can be controlled and the variation range ofβcan be efficiently enlarged by approximately two times(Δβ~36 cm-1).With the CWG method,we successfully realize the most compact Fs LDW directional couplers with a 9μm pitch in a straight-line form and achieve the reconstruction of the Hamiltonian of a Hermitian array.Thus,the study represents a step further toward the fine control of the coupling between waveguides and compact integration of Fs LDW PICs.