In the 2266, we see a couple of bars in a brig on the Enterprise — of course, these particular ones wouldn't do a very good job of keeping the prisoner inside without the force field that is also in place.
After an extensive search, I cannot find an example of a brig or cell on a Federation starship or installation from the 23rd Century or later that does not involve a force field in some crucial way. Any bars that are present are merely part of the electrostatic force field generator and are not positioned in a way that would confine the prisoner when the field is inactive (as in the "Charlie X" example above).
If Cal were sent to a prison, it would likely be the Federation Penal Colony in New Zealand where Tom Paris and other Maquis collaborators were sent:
Apart from the unstylish grey jumpsuits, this seems to be a fairly comfortable open-air prison. No bars in sight.
In short, it seems that force-field-based brigs and open-plan penal colonies are standard in the Federation of the 23rd and 24th centuries. This seems to be consistent with the general ethos of the Federation.
By then, "behind bars" is probably just a figure of speech, like "hang up the phone" or "dial the number" or "make sure you get that on tape".