  We demonstrate a simple projective measurement based on the quantum eraser
concept that can be used to characterize the disturbances of any communication
channel. Quantum erasers are commonly implemented as spatially separated path
interferometric schemes. Here we exploit the advantages of redefining the
which-path information in terms of spatial modes, replacing physical paths with
abstract paths of orbital angular momentum (OAM). Remarkably, vector modes
(natural modes of free-space and fiber) have a non-separable feature of
spin-orbit coupled states, equivalent to the description of two independently
marked paths. We explore the effects of fiber perturbations by probing a
step-index optical fiber channel with a vector mode, relevant to high-order
spatial mode encoding of information for ultra-fast fiber communications.
