Most standard SSPDs have active areas ranging from 10µm to 50µm. While efficient, these small areas require precise alignment and are sensitive to beam wander (shifts in the light beam's position).
I notice you're asking for a deep piece on "sspd 137" — but that term isn't widely recognized in major public databases, academic literature, or legal/technical references I have access to. It's possible that: sspd 137