Hi. I am interested in medical physics, but I also like to become a laser scientist. If I have a masters in medical physics can I go straight to laser physics Phd, if not what better option can I take after having master in medical physics.
It depends of course on which kind of research you want to do, but I think that being a medical physics you should have already some kind of formation on laser physics, so it could be not so impossible to find a PhD position on that. Anyway the best way can be trying to apply (they usually look at your background when they make a decision) or even better try to contact some professor in the field you are interested in... look at Universities websites, find some professor whose research interests overlap with yours, consider if you could like to go to that University and then contact directly the professor... he is the best person which can indicate you if you are suitable and in case the procedure you have to follow to apply for the PhD. Also the procedure will depend on where you are =)
In most cases a master's degree in medical physics is a professional degree that is intended for preparing the student to work as a clinical medical physicist. The amount of research involved really depends on the specifics of the program. Some are full thesis-based MSc programs, while others are more course-based with some kind of a formal project (and yes sometimes the differences there are simply semantics).
If you were to complete a medical physics MSc there is no reason why you couldn't apply for a PhD in another field. However, it may not be as likely to bolster your chances of acceptance as, for example, doing an MSc in that other field. You might want to think of it more like going to medical school once you're done your undergrad and then returning to academia to do a PhD. (Although, maybe that's not the best analogy. You're still doing some physics, even in the most clinically-oriented MSc program.)
Most often though, once students get into the medical physics track, they tend to stay because of the career prospects as compared to other subfields of physics.