scottyjr:
Are you saying that when there is no smoke detection there is no signal, and when there is smoke detection there is the signal you displayed above?
Yes.
Does the device also put put any other signals that you need to identify as NOT being a smoke detection signal?
No.
Then all you need to do is to characterize enough of the pulse train to make sure that detection would be unique. A few sequential on/off times should suffice.