Novel design of monolithically integrated photonic devices: Teardrop laser and polarization rotator-splitter

In this dissertation, we report two novel designs of monolithically integrated photonic devices, a single facet teardrop shape laser which can be used as an on-chip reflector, and a mode-evolution-based polarization rotator-splitter which enables polarization division multiplexing for high-speed optical communications.;The matched bend criterion method for effectively reducing the straight-bend-straight transition loss in planar optical waveguide, is applied to design a single facet teardrop laser utilizing an on-chip loop mirror reflector. Simulations of a matched bends teardrop reflector show a great improvement in power reflectance (97%) over unmatched-bend designs (10%). The teardrop laser is fabricated by deep etching and non-selective sidewall oxidation of an 808 nm AlGaAs/InAlGaAs/GaAs heterostructure. The resulting high-index-contrast ridge waveguide laser exhibits a low threshold current of 43 mA, a high slope efficiency of 0.465 W/A, and a single spectral lasing mode with a side mode suppression ratio of 24 dB.;To reduce the size of an individual teardrop laser to increase the density of matched bend devices integrated on a single chip, a scaling rule is introduced for reducing the dimensions of a matched waveguide bend without inducing additional transition loss. Mathematical expressions describing the wave propagation in the bend are obtained via a combination of analytical analysis and numerical calculations. These expressions show that if the bend radius R shrinks discretely by n=(m1/m2)3 times, where m1 and m2 are the integer number of beat lengths in the original and scaled matched bends, respectively, and the effective width of the waveguide is reduced also by n1/3 times, then the loss of the matched bend (straight-curved-straight waveguide) transition will remain constant.;A mode-evolution-based polarization rotator-splitter built on an InP substrate is also proposed by combining a mode converter and an adiabatic asymmetric Y-coupler. The mode converter, consisting of a bi-level taper and a width taper, effectively converts the fundamental TM mode into the second order TE mode without changing the polarization of the fundamental TE mode. The following adiabatic asymmetric Y-coupler splits the fundamental and the second order TE modes and also converts the second order TE mode into the fundamental TE mode. A shallow etched structure is proposed for the width taper to enhance the polarization conversion efficiency. The device has a total length of 1350 mum, a polarization extinction ratio over 25 dB and an insertion loss below 0.5 dB for both the TE and TM modes, over the entire C+L band wavelength range from 1528 to 1612 nm covering all C+L band. Because the device is designed based on mode evolution principles, it has large fabrication tolerances. The insertion loss remains below 1 dB and the polarization extinction ratio remains over 17 dB with respect to a width variation of +/- 0.12 mum at the wavelength of 1570 nm, or +/- 0.08 mum over the entire C+L band.