  We investigate experimentally and theoretically the Coherent Population
Trapping (CPT) effect occurring in 87Rb D1 line due to the interaction with
linearly polarized laser light (lin||lin CPT). In this configuration the
coherence has a quadrupol like nature and is strongly influenced by the
hyperfine structure of the excited state; consequently the quantum interference
between CPT states is an essential feature of this interaction scheme. We study
the lin||lin CPT signal as a function of the laser optical detuning. The
comparison between experimental and theoretical results allows us to quantify
the contribution from different CPT states to the total signal. Based on these
results we investigate the signal depending on both the pressure broadening of
the optical transition and the laser linewidth, and we find in which conditions
the laser linewidth does not degrade the amplitude of the lin||lin CPT.
