Past exam of the laser science and quantum information processing course of the University of Oxford / 2022 / 2 / b / Solution

Propagate from one beam waist to the other, using the optical lens ray transfer matrix:Optical mode matching requires $q_A=i{z}_A$ to transform to $q_B=i{z}_B$. Multiplying the denominator across givesEquating real and imaginary parts and multiplying by $f$ yieldsSet $\delta =u-f$. The second equation gives $v=f+(z_B/z_A)\delta$. Substituting this into the first cancels the terms $f\delta$:Consequently the two algebraic branches areEquivalently, $u=f+\delta$. The signs in $u$ and $v$ are the same. Real solutions require $\boxed{f^2\ge z_A{z}_B}$; a converging optical lens therefore needs $f\ge \sqrt{z_A{z}_B}$. At equality $u=v=f$. For the stated arrangement, additionally require $u,v\ge 0$ and that the chosen optical lens position is accessible outside both optical resonators. A negative $v$ is an algebraic virtual-waist solution, not a downstream beam waist.