Efficiency enhancement of light coupling and absorption with optical waveguide arrays

One of the presented works investigates a possible solution for general optical power coupling problem arising from width mismatch of in-/out- waveguide channels. The idea is based on gradually converging of light to provide high coupling efficiency and utilizes inhomogeneous medium having a Gaussian refractive index profile. Initially, the graded-index coupler was designed without considering a specific operating wavelength regime at the 2 dimensional domain (employing only normalized units: operating frequency, structural dimensions). Effective medium theory is used to replace spatially varying index profile with dielectric stripes of finite length/width and constant effective refractive index. Finite-difference time-domain method is used to determine the parameters that play a key role to enhance the light coupling efficiency and investigate the sampling effect of designed power coupler. Comparing the coupling performance of conventional benchmark adiabatic and butt couplers with the designed coupler with optical waveguide arrays, coupling efficiency increases from approximately 30% to 95% over a wide frequency interval. Then, to realize realistic optical power coupler appropriate to integrated photonic applications, we continued with 3 dimensional time domain analyses and the proposed structure is numerically designed on a silicon-on-insulator wafer. Equalizing the center wavelength of high transmission window to be equal to 1.55 μm to operate at telecom wavelength regime, the dimensions of the striped coupler becomes 9.77 µm (along the transverse to propagation direction) and 7.68 µm (along the propagation direction). The microwave experiments have done to demonstrate the working principle of the proposed stripped waveguide coupler and to understand the effect of a gradient index modulation on the incident spatial beam width compression. A spatial compression is vital for a coupling operation and is in concordance with the numerically calculated field compression/converging phenomenon. Spot size conversion ratio as high as 7.1:1 is measured, whereas a coupling efficiency over 60% in the frequency range of 5.0-16.0 GHz has been also demonstrated. Finally, the important highlights of proposed coupler with dielectric stripes can be summarized as follows: the structure is compact, is less dispersive comparing to the conventional index modulation based optical couplers, is more feasible to fabricate, and is not exposed to additional intrinsic material losses because of all dielectric. Other one of the presented works, a compact low-cost multicolor germanium infrared photodetector is designed by utilizing the guided-mode resonance condition of a one-dimensional grating with adiabatically tuned in-plane periodicity. It is shown numerically that the resonance phenomenon is accompanied by a spectral splitting, as every wavevector component is matched with different instantaneous local in-plane periods. In result, the proposed photodetector enables multi-wavelength photon absorption up to 61% efficiency and an ideal responsivity up to 0.63 A/W over a 70 nm bandwidth, while maintaining a very small active region of 0.413µm2. The reported results provide a very valuable insight into highly miniaturized ultra-compact multicolor photodetection schemes. Keywords: Optical waveguide arrays, Photonic lattice, Optical coupler, Effective medium theory, Guided-mode rezonance, Optical absorption, Infrared fotodetector.