  The proximity effect, which arises at the interface between two fermionic
superfluids with different critical temperatures, is examined with a non-local
(integral) equation whose kernel contains information about the size of Cooper
pairs that leak across the interface. This integral approach avoids reference
to the boundary conditions at the interface that would be required with a
differential approach. The temperature dependence of the pair penetration depth
on the normal side of the interface is determined over a wide temperature
range, also varying the inter-particle coupling along the BCS-BEC crossover
independently on both sides of the interface. Conditions are found for which
the proximity effect is optimized in terms of the extension of the pair
penetration depth.
