||High efficient optical spot size converter (SSC) has become one of the major elements in optical fiber communications because it enables high coupling efficiency between different mode sizes of waveguides. Large core of single mode fiber is allowed to efficiently couple to small size waveguides, enabling low cost fiber-related package into high performance photonic integration. Among the types of SSCs, tapered optical direction coupler (TODC) has been widely applied to SSC because the resonant condition could bring up strong power coupling between waveguides within a short distance. However, the price is on its wavelength dependence and thus such SSC is not for wavelength division multiplexing (WDM) technique.|
In this work, a new type of SSC formed by cascading different types of TODC is proposed and fabricated, where the converted two modes are a passive waveguide (PW) and active waveguide (AW). PW and AW are formed by alternated InP/InGaAsP layers and InGaAsP multiple quantum well (MQW) sandwiched by InP layers respectively. The different resonance conditions can be placed in different widths of TODC defined by undercut wet etching AW. Through 3 sections of TODC, the SSC is fabricated. The measured far field angles for SSC and EAM are 7.6° (horizontal) ×7.18° (vertical) and 17.5° (horizontal) ×30.9° (vertical), confirming the function of SSC. Using cutback method, the coupling loss between fiber and device is extracted, where the PW has -2.6dB coupling loss with fiber and AW has -6.6dB with tapered fiber. Using AW as electroabsorption integrated with SSC, the photocurrent and optical transmission properties can be used for extracting transfer loss of SSC. By reversed directions of optical transmission, the ratio of the measured photocurrent from EAM is range from 1520nm to 1600nm along the wavelength range of from 1520nm to 1600nm for different directions of pumping, suggesting that the SSC has broadband properties.