Monolithically-integrated semiconductor optical amplifiers (SOAs) have the potential for enabling high-speed and low-crosstalk
optical switches in reconfigurable optical add-drop multiplexers (ROADMs). Using integrated 4x4 switches as
the building blocks for large-scale ROADMs, instead of 2x2 switches, will reduce alignment issues and assembly steps
during manufacturing. The switch is based on SOAs, quantum well intermixed (QWI) passive 1x4 MMI
splitters/combiners, and total internal reflection mirrors. We present the results of the 4x4 switch design, for a switch of
5.3 mm x 3.5 mm in size, with estimated total excess on-chip losses of 23 dB.
To avoid the commonly required regrowth steps in conventional distributed feedback laser fabrication, laterally-coupled
distributed feedback (LC-DFB) lasers lithographically pattern the grating out of the ridge waveguide. Using higher
order gratings increases the lithographic tolerances, resulting in lasers that are more amenable to mass-manufacturing
techniques, such as stepper lithography. We have extended the modified coupled-mode theory to a two-dimensional
cross-section, and thereby identified grating geometries that are both fabrication-tolerant and provide high performance.
We report on our recent progress in polarization control and polarization compensator designs in SOI-based planar
reflective gratings for a range of silicon core thicknesses of 0.1 μm to 10 μm. The dispersion property of the silicon
slab, without a compensator region, was found to limit the applicability of SOI gratings for achieving the polarizationinsensitive
performance of wavelength division multiplexing systems based on planar gratings. We have found that in
coarse wavelength division multiplexing systems, the birefringence of the uncompensated slab becomes impractical at
core thicknesses below 1.7 μm. Our findings clearly show that shallow etched polarization compensators can effectively
eliminate polarization dependence only in thick-core gratings and only in applications requiring free spectral ranges
(FSRs) of no more than 80 nm. In silicon cores with thicknesses of less than 1.0 μm, the significantly different value of
linear dispersion strength for the two polarization states make traditional compensator designs ineffective, since only the
central wavelength can be fully compensated. We used our findings to construct a procedure for building compensators
with a flat polarization response over wide FSRs (>80 nm). The results of our study were applied to the design of a
polarization compensator in an 18-channel multiplexer for use in coarse wavelength division multiplexing. Our
simulation results show that a careful selection of the silicon core thicknesses in the slab and compensator regions is
essential for achieving low-cross talk and low insertion loss devices. The application of thin core planar silicon gratings
to building silicon interconnects is discussed.
This paper describes a monolithically integrated 1x2 SOA-based switch in InGaAsP/InP. It can be fabricated in one epitaxial growth step, has a footprint of only 4.2mm x 0.35mm, operates on sub-ns time scales and is meant to be integrated with other passive and active waveguide devices on the same InP substrate. The design process optimized the device dimensions using a modified finite-element modal-overlap method. This method provides significant computational savings compared to full beam-propagation method (BPM) simulations. The device uses a single-mode vertical integration technique for a monolithic integration of active and passive waveguide components. To compensate for the polarization sensitivity, tensile-strained quantum well active regions are used. To switch a signal to an output waveguide, the SOA in that waveguide is forward-biased while the SOA in the other output waveguide is reverse-biased to provide a large attenuation (>30dB), resulting in minimal crosstalk. This switch has an estimated insertion loss of 4dB, with a polarization dependent loss of < 1dB.
Early development work in the design of optical power splitters, likely influenced by similar construction in the microwave regime, placed heavy emphasis on Y-branch designs with the output waveguides immediately branching from the input waveguide at non-zero angle. This design approach, which is still prevalent, is fundamentally flawed from the perspective of both optical power flow and fabrication, as it leads to significant excess loss and/or a large
statistical variance. If inherent broadband response is not a critical requirement, directional-coupler or multimode-interference splitters are usually chosen instead. We demonstrate, choosing a minimal function perspective where the optical design is sensitive
to the smallest possible set of critical fabrication parameters, that robust and low-loss Y-branch designs are indeed possible. The minimum gap width between waveguides being the critical parameter, we reveal the dependence of the irreducibly simplest design on all
elements of the parameter space, as they relate to the critical one. In so doing, we show that the concept of bending angle is irrelevant.