The only solution I can see would be to somehow configure your Laser 6 to send out a signal every minuet or so. Then, you’d have to reprogram your MCU to wait for any signal. If one doesn't come, for say 2 minuets, and then your sub should blow the ballast.
If you can't configure your Laser 6 to do that, you'd have to build a transmitter that operates on the same frequency that sends the signal, telling your sub that it's in range.