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 n^(1/3) 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 um, 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 um at the wavelength of 1570 nm, or +/- 0.08 um over the entire C+L band.