Unstable resonators

Question 1

When is a resonator unstable

Answer 1

When g params <0 or >1

Question 2

Why would one use an unstable resonator

Answer 2

Get more of the gain medium worken on with a larger beam (even if you slightly loose a little bit due to diffrative effects and instability) think of confocal shape resulting in a large beam

stable resonators produce very small beams which extract only a littl ebit of the avaible gain medium

Question 3

List 3 adv of unstable resonators

Answer 3

Large mode vol
small divernce
no transmissive optics

Question 4

what does the beam look like in near and far field if you use an unstbale resonator

Answer 4

Near field - donut (hole at centre)
far field - airy pattern

more generally we notice that modes of unstable resonators are not eiegen functions of the unlimited fresnel integral and the near and far field are different. in the far field the diffr image fo the annular ring aperture can be seen