I'm brushing up my concepts on LASER's and I was just curious about this. Does a three level pumping scheme necessarily imply that exactly three energy states have to be present? Or can more or or less energy states can be present as well? I know that a four level pumping scheme is more efficient but this is just a question that popped up in my mind. I'm sorry if I got my concepts wrong in some place and would really appreciate it if someone could explain the rationale behind this to clear my confusion. Sorry for taking up your valuable time!
You will need at least three states for lasing to occur (with reasonable probability) but that does not mean that other states cannot be present. If the pump frequencies are not tuned to reach these other states then they will not be occupied with any appreciable probability and hence will not influence the lasing process.