Active Photon Regeneration for ALPS II

ALPS II, the Any Light Particle Search, is a second-generation Light Shining through a Wall experiment that hunts for axion-like particles. It uses two optical cavities; one on each side of the wall, to first generate light particles from a very strong intra-cavity optical field and then turn these particles back into photons in the second cavity called the regeneration cavity. ALPS II will either detect axion-like particles or provide an upper limit on their coupling strength to two photons of $g_{a\gamma\gamma}\leq2\times10^{-11}\text{GeV}^{-1}$. The experiment is currently transitioning from the design and construction phase to the commissioning phase with science runs expected to start in 2021. One of the challenges of ALPS II is that one of the cavities will have to track the length of the other cavity at the pm level. This paper discusses the possibility of replacing the regeneration cavity with an active regeneration system which promises a similar signal to noise ratio. For ALPS II, this is a risk reduction activity. However, the regeneration cavity in ALPS is fundamentally very similar to signal recycling in interferometric gravitational wave detectors (LIGO, VIRGO, GEO, and KAGRA) and the basic idea might very well be applicable there as well.