We or "the authors"?
I don't think it makes much of a difference; both are used widely. Some people like to write their names as well, eg. J. Doe may write, 'Doe et al [22] demonstrated...'.

The last one is important if you're submitting to a double-blind review. Between the first two, it  depends on how much attention you want to draw to the previous work- it could be of prime significance, or it may be an offhand, supporting reference. In the first case, you may like to use 'we' to establish your group as an important contributor in the field. Nothing hard and fast, it's quite subjective and a matter of style- most people would consider 'the authors demonstrated' to be a more formal expression.