I've found an SE question (Laser without a cavity) about that, but it isn't exactly the same as mine.
My thinking is: no, you can't, because without the cavity, there wouldn't be possibility for one dominant phase to take over and create stimulated emission for that particular phase only.
Best you could achieve is just very strong, kinda-monochromatic light amplification. Am I right?
But what would be other properties of such light source? Would it create a beam? I don't think so, because lack of mirrors would make it impossible for one direction photons to take over the stimulated emission. Is that correct?
Best Answer
Masers, and even optical frequency lasers, occur in molecular clouds. These are single-pass processes rather than resonant cavities and so not always regarded as a "true" laser. See: https://en.wikipedia.org/wiki/Astrophysical_maser