Tapered Velocity Couplers and Devices: a Treatise

A polarization independent device is highly desirable for use in single-mode fiber optical communication systems. Tapered velocity coupler (TVC) is expected to play an important role since its operation is polarization independent as well as wavelength insensitive. Thus far, TVC has received little attention primarily because of the unusually long device length required for complete power transfer. In this dissertation we establish that a TVC with an acceptable device length for integration can be indeed realized and integrated by tapering in index as well as in dimension. We demonstrate, for the first time, that complete power transfer can be achieved in a tapered, both in index and in dimension, velocity coupler in Ti:LiNbO _3 with device length reduced to one quarter of that of conventional TVC. The coupler is analyzed by use of step transition model in conjunction with local normal modes of the grade index TVC, overcoming the deficiency of the five-layer step index model. We further demonstrate a Ti:LiNbO_3 digital optical switch with the smallest voltage length product reported to date, namely, 7.2 Vcm for TM and 24 Vcm TE mode with a 15 dB cross talk. In an effort to extend the tapered, both in index and in dimension, velocity coupler concepts to step index compound semiconductor waveguides, we introduce proton exchanged periodically segmented (PEPS) waveguides. PEPS waveguides in LiNbO_3 are first studied theoretically and experimentally. The mode index of PEPS waveguides increases linearly and saturates finally with increase of duty cycle. Next, segmented waveguides in AlGaAs/GaAs are characterized in terms of propagation loss and modal size with respect to duty cycle. These segmented waveguides will be utilized in the development of step index tapered velocity couplers. Finally, we present an application for TVC as an optical interconnect. In particular, a tapered waveguide interconnect between a single quantum well (SQW) laser and a multi-quantum well (MQW) modulator is presented. Using vertical coupling between SQW guiding layers and a tapered, both in index and dimension, MQW layer, the tapered interconnect is modified for complete power transfer from SQW laser to MQW modulator.