Thanks for the replies.
The idea I have been considering regarding the Laser comms system is that we still utilise the tried and trusted ROV umbilical.
The main reason for keeping it, is that if all fails, we still have a rudimentary retrieval system to get the ROV back topside.
However, I’m a great believer in failsafe, and reducing failure, whether it is in components or risk.
The general idea is that the ROV end of the umbilical terminates with a Laser & photodiode housing, this housing has a simple connection to a translucent window on the hull of the ROV, data is transmitted and received through the translucent window, thereby removing a through hull connection. If the laser housing can be encased in polycarbonate, this would give us a watertight housing that could be machined.
If the ROV umbilical end housing onto the ROV hull has a simple connection that can be released in the event of an emergency i.e. comms failure, power failure, umbilical snag/entrapment, then we have a method of detaching the ROV from its umbilical and allow it to free access to topside. A small CO2 cartridge and stored SMB (as used by divers) or bladder that can be activated by trigger mechanism that could disconnect the umbilical as it is deployed.
Just food for thought, as with most projects, we could end up over engineering/complicating the project.