  Nonlocality of two-mode states of light is addressed by means of CHSH
inequality based on displaced on/off photodetection. Effects due to non-unit
quantum efficiency and nonzero dark counts are taken into account. Nonlocality
of both balanced and unbalanced superpositions of few photon-number states, as
well as that of multiphoton twin beams, is investigated. We find that
unbalanced superpositions show larger nonlocality than balanced one when noise
affects the photodetection process. De-Gaussification by means of
(inconclusive) photon subtraction is shown to enhance nonlocality of twin beams
in the low energy regime. We also show that when the measurement is described
by a POVM, rather than a set of projectors, the maximum achievable value of the
Bell parameter in the CHSH inequality is decreased, and is no longer given by
the Cirel'son bound.
