  Managing insertion losses, polarizations and device footprint is crucial in
developing large-scale photonic integrated circuits (PICs). This paper presents
a solution to these critical challenges by designing a semiconductor optical
amplifier (SOA) in the O-band with reduced polarization sensitivity, leveraging
the ultra-compact InP Membrane on Silicon (IMOS) platform. The platform is
compatible with close integration atop electronics, via densely populated
vertical interconnects. The SOA incorporates a thin tensile-strained bulk
active layer to mitigate polarization sensitivity. The developed 500 um long
SOA has a peak gain of 11.5 dB at 1350 nm and an optimal polarization
dependency of less than 1 dB across a 25 nm bandwidth, ranging from 1312 nm to
1337 nm. The device is practical for integrated circuits where multiple
amplifiers work in cascades with a minimal 6.5 dB noise figure (NF) measured at
the gain peak. The designed vertical active-passive transition, achieved
through inverse tapering, allows for effective field coupling in the vertical
direction resulting in a transmission efficiency of over 95% at the transition
and minimal polarization sensitivity of less than 3%. The device yields
significant gain at a small current density of less than 3 kA/cm2 as the result
of minimalist gain medium structure, reducing joule heating and improving
energy efficiency. This is especially relevant in applications such as optical
switching, where multiple SOAs populate the PIC within a small area.
Consequently, the simulated and fabricated low polarization sensitive O-band
SOA is a suitable candidate for integration into large-scale, ultra-compact
photonic integrated circuits.
