  It has recently been shown [Rossi et al., Phys. Rev. Lett. 119, 123603
(2017); ibid. 120, 073601 (2018)] that feedback--controlled in--loop light can
be used to enhance the efficiency of optomechanical systems. We analyse the
theoretical ground at the basis of this approach and explore its potentialities
and limitations. We discuss the validity of the model, analyse the properties
of in-loop cavities and we show how they can be used to observe coherent
optomechanical oscillations also with weakly coupled system, improve the
sideband cooling performance, and increase ponderomotive squeezing.
